diff --git a/docs/grammar/Solidity.g4 b/docs/grammar/Solidity.g4
index 3d66df9b6..bd2d1ad5a 100644
--- a/docs/grammar/Solidity.g4
+++ b/docs/grammar/Solidity.g4
@@ -19,6 +19,7 @@ sourceUnit: (
 	| constantVariableDeclaration
 	| structDefinition
 	| enumDefinition
+	| errorDefinition
 )* EOF;
 
 //@doc: inline
@@ -90,6 +91,7 @@ contractBodyElement:
 	| enumDefinition
 	| stateVariableDeclaration
 	| eventDefinition
+	| errorDefinition
 	| usingDirective;
 //@doc:inline
 namedArgument: name=identifier Colon value=expression;
@@ -289,6 +291,18 @@ eventDefinition:
 	Anonymous?
 	Semicolon;
 
+/**
+ * Parameter of an error.
+ */
+errorParameter: type=typeName name=identifier?;
+/**
+ * Definition of an error.
+ */
+errorDefinition:
+	Error name=identifier
+	LParen (parameters+=errorParameter (Comma parameters+=errorParameter)*)? RParen
+	Semicolon;
+
 /**
  * Using directive to bind library functions to types.
  * Can occur within contracts and libraries.
@@ -365,9 +379,9 @@ tupleExpression: LParen (expression? ( Comma expression?)* ) RParen;
 inlineArrayExpression: LBrack (expression ( Comma expression)* ) RBrack;
 
 /**
- * Besides regular non-keyword Identifiers, the 'from' keyword can also occur as identifier outside of import statements.
+ * Besides regular non-keyword Identifiers, some keywords like 'from' and 'error' can also be used as identifiers.
  */
-identifier: Identifier | From;
+identifier: Identifier | From | Error;
 
 literal: stringLiteral | numberLiteral | booleanLiteral | hexStringLiteral | unicodeStringLiteral;
 booleanLiteral: True | False;
diff --git a/docs/grammar/SolidityLexer.g4 b/docs/grammar/SolidityLexer.g4
index d8e89b278..560d78e6a 100644
--- a/docs/grammar/SolidityLexer.g4
+++ b/docs/grammar/SolidityLexer.g4
@@ -29,12 +29,13 @@ Do: 'do';
 Else: 'else';
 Emit: 'emit';
 Enum: 'enum';
+Error: 'error'; // not a real keyword
 Event: 'event';
 External: 'external';
 Fallback: 'fallback';
 False: 'false';
 Fixed: 'fixed' | ('fixed' [1-9][0-9]* 'x' [1-9][0-9]*);
-From: 'from';
+From: 'from'; // not a real keyword
 /**
  * Bytes types of fixed length.
  */
diff --git a/libsolidity/analysis/ContractLevelChecker.cpp b/libsolidity/analysis/ContractLevelChecker.cpp
index 3bdf128a9..aece2f8cb 100644
--- a/libsolidity/analysis/ContractLevelChecker.cpp
+++ b/libsolidity/analysis/ContractLevelChecker.cpp
@@ -25,10 +25,10 @@
 #include <libsolidity/ast/AST.h>
 #include <libsolidity/ast/TypeProvider.h>
 #include <libsolidity/analysis/TypeChecker.h>
+#include <libsolutil/FunctionSelector.h>
 #include <liblangutil/ErrorReporter.h>
 #include <boost/range/adaptor/reversed.hpp>
 
-
 using namespace std;
 using namespace solidity;
 using namespace solidity::langutil;
@@ -433,6 +433,24 @@ void ContractLevelChecker::checkHashCollisions(ContractDefinition const& _contra
 			);
 		hashes.insert(hash);
 	}
+
+	map<uint32_t, SourceLocation> errorHashes;
+	for (ErrorDefinition const* error: _contract.interfaceErrors())
+	{
+		if (!error->functionType(true)->interfaceFunctionType())
+			// This will result in an error later on, so we can ignore it here.
+			continue;
+		uint32_t hash = selectorFromSignature32(error->functionType(true)->externalSignature());
+		if (errorHashes.count(hash))
+			m_errorReporter.typeError(
+				4883_error,
+				_contract.location(),
+				SecondarySourceLocation{}.append("This error has the same selector: "s, errorHashes[hash]),
+				"Error signature hash collision for " + error->functionType(true)->externalSignature()
+			);
+		else
+			errorHashes[hash] = error->location();
+	}
 }
 
 void ContractLevelChecker::checkLibraryRequirements(ContractDefinition const& _contract)
diff --git a/libsolidity/analysis/DeclarationContainer.cpp b/libsolidity/analysis/DeclarationContainer.cpp
index 25bd37b9f..499c26ff1 100644
--- a/libsolidity/analysis/DeclarationContainer.cpp
+++ b/libsolidity/analysis/DeclarationContainer.cpp
@@ -124,7 +124,7 @@ bool DeclarationContainer::registerDeclaration(
 		// Do not warn about shadowing for structs and enums because their members are
 		// not accessible without prefixes. Also do not warn about event parameters
 		// because they do not participate in any proper scope.
-		bool special = _declaration.scope() && (_declaration.isStructMember() || _declaration.isEnumValue() || _declaration.isEventParameter());
+		bool special = _declaration.scope() && (_declaration.isStructMember() || _declaration.isEnumValue() || _declaration.isEventOrErrorParameter());
 		if (m_enclosingContainer && !special)
 			m_homonymCandidates.emplace_back(*_name, _location ? _location : &_declaration.location());
 	}
diff --git a/libsolidity/analysis/DeclarationTypeChecker.cpp b/libsolidity/analysis/DeclarationTypeChecker.cpp
index d54c22ef3..a6e3b40bf 100644
--- a/libsolidity/analysis/DeclarationTypeChecker.cpp
+++ b/libsolidity/analysis/DeclarationTypeChecker.cpp
@@ -368,7 +368,7 @@ void DeclarationTypeChecker::endVisit(VariableDeclaration const& _variable)
 	}
 
 	// Find correct data location.
-	if (_variable.isEventParameter())
+	if (_variable.isEventOrErrorParameter())
 	{
 		solAssert(varLoc == Location::Unspecified, "");
 		typeLoc = DataLocation::Memory;
diff --git a/libsolidity/analysis/DocStringAnalyser.cpp b/libsolidity/analysis/DocStringAnalyser.cpp
index dba059fd2..018c10f58 100644
--- a/libsolidity/analysis/DocStringAnalyser.cpp
+++ b/libsolidity/analysis/DocStringAnalyser.cpp
@@ -158,6 +158,13 @@ bool DocStringAnalyser::visit(EventDefinition const& _event)
 	return true;
 }
 
+bool DocStringAnalyser::visit(ErrorDefinition const& _error)
+{
+	handleCallable(_error, _error, _error.annotation());
+
+	return true;
+}
+
 void DocStringAnalyser::handleCallable(
 	CallableDeclaration const& _callable,
 	StructurallyDocumented const& _node,
diff --git a/libsolidity/analysis/DocStringAnalyser.h b/libsolidity/analysis/DocStringAnalyser.h
index e13cab898..1fa076359 100644
--- a/libsolidity/analysis/DocStringAnalyser.h
+++ b/libsolidity/analysis/DocStringAnalyser.h
@@ -43,6 +43,7 @@ private:
 	bool visit(VariableDeclaration const& _variable) override;
 	bool visit(ModifierDefinition const& _modifier) override;
 	bool visit(EventDefinition const& _event) override;
+	bool visit(ErrorDefinition const& _error) override;
 
 	CallableDeclaration const* resolveInheritDoc(
 		std::set<CallableDeclaration const*> const& _baseFunctions,
diff --git a/libsolidity/analysis/DocStringTagParser.cpp b/libsolidity/analysis/DocStringTagParser.cpp
index cb3c0925c..30ecfaf2f 100644
--- a/libsolidity/analysis/DocStringTagParser.cpp
+++ b/libsolidity/analysis/DocStringTagParser.cpp
@@ -92,6 +92,13 @@ bool DocStringTagParser::visit(EventDefinition const& _event)
 	return true;
 }
 
+bool DocStringTagParser::visit(ErrorDefinition const& _error)
+{
+	handleCallable(_error, _error, _error.annotation());
+
+	return true;
+}
+
 void DocStringTagParser::checkParameters(
 	CallableDeclaration const& _callable,
 	StructurallyDocumented const& _node,
@@ -134,11 +141,14 @@ void DocStringTagParser::handleCallable(
 )
 {
 	static set<string> const validEventTags = set<string>{"dev", "notice", "return", "param"};
+	static set<string> const validErrorTags = set<string>{"dev", "notice", "param"};
 	static set<string> const validModifierTags = set<string>{"dev", "notice", "param", "inheritdoc"};
 	static set<string> const validTags = set<string>{"dev", "notice", "return", "param", "inheritdoc"};
 
 	if (dynamic_cast<EventDefinition const*>(&_callable))
 		parseDocStrings(_node, _annotation, validEventTags, "events");
+	else if (dynamic_cast<ErrorDefinition const*>(&_callable))
+		parseDocStrings(_node, _annotation, validErrorTags, "errors");
 	else if (dynamic_cast<ModifierDefinition const*>(&_callable))
 		parseDocStrings(_node, _annotation, validModifierTags, "modifiers");
 	else
diff --git a/libsolidity/analysis/DocStringTagParser.h b/libsolidity/analysis/DocStringTagParser.h
index 41dcca5df..548de1d42 100644
--- a/libsolidity/analysis/DocStringTagParser.h
+++ b/libsolidity/analysis/DocStringTagParser.h
@@ -44,6 +44,7 @@ private:
 	bool visit(VariableDeclaration const& _variable) override;
 	bool visit(ModifierDefinition const& _modifier) override;
 	bool visit(EventDefinition const& _event) override;
+	bool visit(ErrorDefinition const& _error) override;
 
 	void checkParameters(
 		CallableDeclaration const& _callable,
diff --git a/libsolidity/analysis/PostTypeChecker.cpp b/libsolidity/analysis/PostTypeChecker.cpp
index cf2a39746..329408832 100644
--- a/libsolidity/analysis/PostTypeChecker.cpp
+++ b/libsolidity/analysis/PostTypeChecker.cpp
@@ -23,6 +23,7 @@
 #include <liblangutil/ErrorReporter.h>
 #include <liblangutil/SemVerHandler.h>
 #include <libsolutil/Algorithms.h>
+#include <libsolutil/FunctionSelector.h>
 
 #include <boost/range/adaptor/map.hpp>
 #include <memory>
@@ -70,6 +71,11 @@ void PostTypeChecker::endVisit(VariableDeclaration const& _variable)
 	callEndVisit(_variable);
 }
 
+void PostTypeChecker::endVisit(ErrorDefinition const& _error)
+{
+	callEndVisit(_error);
+}
+
 bool PostTypeChecker::visit(EmitStatement const& _emit)
 {
 	return callVisit(_emit);
@@ -362,6 +368,33 @@ private:
 	/// Flag indicating whether we are currently inside a StructDefinition.
 	int m_insideStruct = 0;
 };
+
+struct ReservedErrorSelector: public PostTypeChecker::Checker
+{
+	ReservedErrorSelector(ErrorReporter& _errorReporter):
+		Checker(_errorReporter)
+	{}
+
+	void endVisit(ErrorDefinition const& _error) override
+	{
+		if (_error.name() == "Error" || _error.name() == "Panic")
+			m_errorReporter.syntaxError(
+				1855_error,
+				_error.location(),
+				"The built-in errors \"Error\" and \"Panic\" cannot be re-defined."
+			);
+		else
+		{
+			uint32_t selector = selectorFromSignature32(_error.functionType(true)->externalSignature());
+			if (selector == 0 || ~selector == 0)
+				m_errorReporter.syntaxError(
+					2855_error,
+					_error.location(),
+					"The selector 0x" + toHex(toCompactBigEndian(selector, 4)) + " is reserved. Please rename the error to avoid the collision."
+				);
+		}
+	}
+};
 }
 
 PostTypeChecker::PostTypeChecker(langutil::ErrorReporter& _errorReporter): m_errorReporter(_errorReporter)
@@ -371,4 +404,5 @@ PostTypeChecker::PostTypeChecker(langutil::ErrorReporter& _errorReporter): m_err
 	m_checkers.push_back(make_shared<ModifierContextChecker>(_errorReporter));
 	m_checkers.push_back(make_shared<EventOutsideEmitChecker>(_errorReporter));
 	m_checkers.push_back(make_shared<NoVariablesInInterfaceChecker>(_errorReporter));
+	m_checkers.push_back(make_shared<ReservedErrorSelector>(_errorReporter));
 }
diff --git a/libsolidity/analysis/PostTypeChecker.h b/libsolidity/analysis/PostTypeChecker.h
index fd94785e1..f35330fa1 100644
--- a/libsolidity/analysis/PostTypeChecker.h
+++ b/libsolidity/analysis/PostTypeChecker.h
@@ -40,6 +40,7 @@ namespace solidity::frontend
  *  - whether a modifier is in a function header
  *  - whether an event is used outside of an emit statement
  *  - whether a variable is declared in a interface
+ *  - whether an error uses a reserved signature
  *
  *  When adding a new checker, make sure a visitor that forwards calls that your
  *  checker uses exists in PostTypeChecker. Add missing ones.
@@ -77,6 +78,8 @@ private:
 	bool visit(VariableDeclaration const& _variable) override;
 	void endVisit(VariableDeclaration const& _variable) override;
 
+	void endVisit(ErrorDefinition const& _error) override;
+
 	bool visit(EmitStatement const& _emit) override;
 	void endVisit(EmitStatement const& _emit) override;
 
diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index c3309600b..8c99c8396 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -42,6 +42,7 @@
 
 #include <range/v3/view/zip.hpp>
 #include <range/v3/view/drop_exactly.hpp>
+#include <range/v3/algorithm/count_if.hpp>
 
 #include <memory>
 #include <vector>
@@ -686,30 +687,12 @@ void TypeChecker::visitManually(
 bool TypeChecker::visit(EventDefinition const& _eventDef)
 {
 	solAssert(_eventDef.visibility() > Visibility::Internal, "");
-	unsigned numIndexed = 0;
-	for (ASTPointer<VariableDeclaration> const& var: _eventDef.parameters())
-	{
-		if (var->isIndexed())
-			numIndexed++;
-		if (type(*var)->containsNestedMapping())
-			m_errorReporter.typeError(
-				3448_error,
-				var->location(),
-				"Type containing a (nested) mapping is not allowed as event parameter type."
-			);
-		if (!type(*var)->interfaceType(false))
-			m_errorReporter.typeError(3417_error, var->location(), "Internal or recursive type is not allowed as event parameter type.");
-		if (
-			!useABICoderV2() &&
-			!typeSupportedByOldABIEncoder(*type(*var), false /* isLibrary */)
-		)
-			m_errorReporter.typeError(
-				3061_error,
-				var->location(),
-				"This type is only supported in ABI coder v2. "
-				"Use \"pragma abicoder v2;\" to enable the feature."
-			);
-	}
+	checkErrorAndEventParameters(_eventDef);
+
+	auto numIndexed = ranges::count_if(
+		_eventDef.parameters(),
+		[](ASTPointer<VariableDeclaration> const& var) { return var->isIndexed(); }
+	);
 	if (_eventDef.isAnonymous() && numIndexed > 4)
 		m_errorReporter.typeError(8598_error, _eventDef.location(), "More than 4 indexed arguments for anonymous event.");
 	else if (!_eventDef.isAnonymous() && numIndexed > 3)
@@ -717,6 +700,13 @@ bool TypeChecker::visit(EventDefinition const& _eventDef)
 	return true;
 }
 
+bool TypeChecker::visit(ErrorDefinition const& _errorDef)
+{
+	solAssert(_errorDef.visibility() > Visibility::Internal, "");
+	checkErrorAndEventParameters(_errorDef);
+	return true;
+}
+
 void TypeChecker::endVisit(FunctionTypeName const& _funType)
 {
 	FunctionType const& fun = dynamic_cast<FunctionType const&>(*_funType.annotation().type);
@@ -2279,7 +2269,8 @@ void TypeChecker::typeCheckFunctionGeneralChecks(
 		_functionType->kind() == FunctionType::Kind::DelegateCall ||
 		_functionType->kind() == FunctionType::Kind::External ||
 		_functionType->kind() == FunctionType::Kind::Creation ||
-		_functionType->kind() == FunctionType::Kind::Event;
+		_functionType->kind() == FunctionType::Kind::Event ||
+		_functionType->kind() == FunctionType::Kind::Error;
 
 	if (callRequiresABIEncoding && !useABICoderV2())
 	{
@@ -3426,6 +3417,32 @@ void TypeChecker::endVisit(UsingForDirective const& _usingFor)
 		);
 }
 
+void TypeChecker::checkErrorAndEventParameters(CallableDeclaration const& _callable)
+{
+	string kind = dynamic_cast<EventDefinition const*>(&_callable) ? "event" : "error";
+	for (ASTPointer<VariableDeclaration> const& var: _callable.parameters())
+	{
+		if (type(*var)->containsNestedMapping())
+			m_errorReporter.typeError(
+				3448_error,
+				var->location(),
+				"Type containing a (nested) mapping is not allowed as " + kind + " parameter type."
+			);
+		if (!type(*var)->interfaceType(false))
+			m_errorReporter.typeError(3417_error, var->location(), "Internal or recursive type is not allowed as " + kind + " parameter type.");
+		if (
+			!useABICoderV2() &&
+			!typeSupportedByOldABIEncoder(*type(*var), false /* isLibrary */)
+		)
+			m_errorReporter.typeError(
+				3061_error,
+				var->location(),
+				"This type is only supported in ABI coder v2. "
+				"Use \"pragma abicoder v2;\" to enable the feature."
+			);
+	}
+}
+
 bool TypeChecker::contractDependenciesAreCyclic(
 	ContractDefinition const& _contract,
 	std::set<ContractDefinition const*> const& _seenContracts
diff --git a/libsolidity/analysis/TypeChecker.h b/libsolidity/analysis/TypeChecker.h
index cc8da6f7f..efbf99ada 100644
--- a/libsolidity/analysis/TypeChecker.h
+++ b/libsolidity/analysis/TypeChecker.h
@@ -125,6 +125,7 @@ private:
 	/// case this is a base constructor call.
 	void visitManually(ModifierInvocation const& _modifier, std::vector<ContractDefinition const*> const& _bases);
 	bool visit(EventDefinition const& _eventDef) override;
+	bool visit(ErrorDefinition const& _errorDef) override;
 	void endVisit(FunctionTypeName const& _funType) override;
 	bool visit(InlineAssembly const& _inlineAssembly) override;
 	bool visit(IfStatement const& _ifStatement) override;
@@ -153,6 +154,8 @@ private:
 	void endVisit(Literal const& _literal) override;
 	void endVisit(UsingForDirective const& _usingForDirective) override;
 
+	void checkErrorAndEventParameters(CallableDeclaration const& _callable);
+
 	bool contractDependenciesAreCyclic(
 		ContractDefinition const& _contract,
 		std::set<ContractDefinition const*> const& _seenContracts = std::set<ContractDefinition const*>()
diff --git a/libsolidity/ast/AST.cpp b/libsolidity/ast/AST.cpp
index 1967064b2..44acbe2f0 100644
--- a/libsolidity/ast/AST.cpp
+++ b/libsolidity/ast/AST.cpp
@@ -174,6 +174,15 @@ vector<EventDefinition const*> const& ContractDefinition::interfaceEvents() cons
 	});
 }
 
+vector<ErrorDefinition const*> ContractDefinition::interfaceErrors() const
+{
+	set<ErrorDefinition const*, CompareByID> result;
+	// TODO add all referenced errors
+	for (ContractDefinition const* contract: annotation().linearizedBaseContracts)
+		result += filteredNodes<ErrorDefinition>(contract->m_subNodes);
+	return convertContainer<vector<ErrorDefinition const*>>(move(result));
+}
+
 vector<pair<util::FixedHash<4>, FunctionTypePointer>> const& ContractDefinition::interfaceFunctionList(bool _includeInheritedFunctions) const
 {
 	return m_interfaceFunctionList[_includeInheritedFunctions].init([&]{
@@ -462,6 +471,24 @@ EventDefinitionAnnotation& EventDefinition::annotation() const
 	return initAnnotation<EventDefinitionAnnotation>();
 }
 
+Type const* ErrorDefinition::type() const
+{
+	return TypeProvider::function(*this);
+}
+
+FunctionTypePointer ErrorDefinition::functionType(bool _internal) const
+{
+	if (_internal)
+		return TypeProvider::function(*this);
+	else
+		return nullptr;
+}
+
+ErrorDefinitionAnnotation& ErrorDefinition::annotation() const
+{
+	return initAnnotation<ErrorDefinitionAnnotation>();
+}
+
 SourceUnit const& Scopable::sourceUnit() const
 {
 	ASTNode const* s = scope();
@@ -504,10 +531,10 @@ bool Declaration::isStructMember() const
 	return dynamic_cast<StructDefinition const*>(scope());
 }
 
-bool Declaration::isEventParameter() const
+bool Declaration::isEventOrErrorParameter() const
 {
 	solAssert(scope(), "");
-	return dynamic_cast<EventDefinition const*>(scope());
+	return dynamic_cast<EventDefinition const*>(scope()) || dynamic_cast<ErrorDefinition const*>(scope());
 }
 
 DeclarationAnnotation& Declaration::annotation() const
@@ -653,7 +680,7 @@ set<VariableDeclaration::Location> VariableDeclaration::allowedDataLocations() c
 {
 	using Location = VariableDeclaration::Location;
 
-	if (!hasReferenceOrMappingType() || isStateVariable() || isEventParameter())
+	if (!hasReferenceOrMappingType() || isStateVariable() || isEventOrErrorParameter())
 		return set<Location>{ Location::Unspecified };
 	else if (isCallableOrCatchParameter())
 	{
diff --git a/libsolidity/ast/AST.h b/libsolidity/ast/AST.h
index d546a029f..1f36875e5 100644
--- a/libsolidity/ast/AST.h
+++ b/libsolidity/ast/AST.h
@@ -272,7 +272,7 @@ public:
 	/// @returns true if this is a declaration of a struct member.
 	bool isStructMember() const;
 	/// @returns true if this is a declaration of a parameter of an event.
-	bool isEventParameter() const;
+	bool isEventOrErrorParameter() const;
 
 	/// @returns the type of expressions referencing this declaration.
 	/// This can only be called once types of variable declarations have already been resolved.
@@ -513,6 +513,9 @@ public:
 	std::vector<FunctionDefinition const*> definedFunctions() const { return filteredNodes<FunctionDefinition>(m_subNodes); }
 	std::vector<EventDefinition const*> events() const { return filteredNodes<EventDefinition>(m_subNodes); }
 	std::vector<EventDefinition const*> const& interfaceEvents() const;
+	/// @returns all errors defined in this contract or any base contract
+	/// and all errors referenced during execution.
+	std::vector<ErrorDefinition const*> interfaceErrors() const;
 	bool isInterface() const { return m_contractKind == ContractKind::Interface; }
 	bool isLibrary() const { return m_contractKind == ContractKind::Library; }
 
@@ -740,7 +743,7 @@ private:
 
 /**
  * Base class for all nodes that define function-like objects, i.e. FunctionDefinition,
- * EventDefinition and ModifierDefinition.
+ * EventDefinition, ErrorDefinition and ModifierDefinition.
  */
 class CallableDeclaration: public Declaration, public VariableScope
 {
@@ -1159,6 +1162,47 @@ private:
 	bool m_anonymous = false;
 };
 
+/**
+ * Definition of an error type usable in ``revert(MyError(x))``, ``require(condition, MyError(x))``
+ * and ``catch MyError(_x)``.
+ */
+class ErrorDefinition: public CallableDeclaration, public StructurallyDocumented, public ScopeOpener
+{
+public:
+	ErrorDefinition(
+		int64_t _id,
+		SourceLocation const& _location,
+		ASTPointer<ASTString> const& _name,
+		SourceLocation _nameLocation,
+		ASTPointer<StructuredDocumentation> const& _documentation,
+		ASTPointer<ParameterList> const& _parameters
+	):
+		CallableDeclaration(_id, _location, _name, std::move(_nameLocation), Visibility::Default, _parameters),
+		StructurallyDocumented(_documentation)
+	{
+	}
+
+	void accept(ASTVisitor& _visitor) override;
+	void accept(ASTConstVisitor& _visitor) const override;
+
+	Type const* type() const override;
+
+	FunctionTypePointer functionType(bool _internal) const override;
+
+	bool isVisibleInDerivedContracts() const override { return true; }
+	bool isVisibleViaContractTypeAccess() const override { return true; }
+
+	ErrorDefinitionAnnotation& annotation() const override;
+
+	CallableDeclaration const& resolveVirtual(
+		ContractDefinition const&,
+		ContractDefinition const*
+	) const override
+	{
+		return *this;
+	}
+};
+
 /**
  * Pseudo AST node that is used as declaration for "this", "msg", "tx", "block" and the global
  * functions when such an identifier is encountered. Will never have a valid location in the source code
diff --git a/libsolidity/ast/ASTAnnotations.h b/libsolidity/ast/ASTAnnotations.h
index 6c19f3dc5..c4212e201 100644
--- a/libsolidity/ast/ASTAnnotations.h
+++ b/libsolidity/ast/ASTAnnotations.h
@@ -184,6 +184,11 @@ struct EventDefinitionAnnotation: CallableDeclarationAnnotation, StructurallyDoc
 {
 };
 
+struct ErrorDefinitionAnnotation: CallableDeclarationAnnotation, StructurallyDocumentedAnnotation
+{
+};
+
+
 struct ModifierDefinitionAnnotation: CallableDeclarationAnnotation, StructurallyDocumentedAnnotation
 {
 };
diff --git a/libsolidity/ast/ASTForward.h b/libsolidity/ast/ASTForward.h
index 80d37d399..d413c9e71 100644
--- a/libsolidity/ast/ASTForward.h
+++ b/libsolidity/ast/ASTForward.h
@@ -57,6 +57,7 @@ class VariableDeclaration;
 class ModifierDefinition;
 class ModifierInvocation;
 class EventDefinition;
+class ErrorDefinition;
 class MagicVariableDeclaration;
 class TypeName;
 class ElementaryTypeName;
diff --git a/libsolidity/ast/ASTJsonConverter.cpp b/libsolidity/ast/ASTJsonConverter.cpp
index d62445081..40f72bde7 100644
--- a/libsolidity/ast/ASTJsonConverter.cpp
+++ b/libsolidity/ast/ASTJsonConverter.cpp
@@ -481,6 +481,17 @@ bool ASTJsonConverter::visit(EventDefinition const& _node)
 	return false;
 }
 
+bool ASTJsonConverter::visit(ErrorDefinition const& _node)
+{
+	setJsonNode(_node, "ErrorDefinition", {
+		make_pair("name", _node.name()),
+		make_pair("nameLocation", sourceLocationToString(_node.nameLocation())),
+		make_pair("documentation", _node.documentation() ? toJson(*_node.documentation()) : Json::nullValue),
+		make_pair("parameters", toJson(_node.parameterList()))
+	});
+	return false;
+}
+
 bool ASTJsonConverter::visit(ElementaryTypeName const& _node)
 {
 	std::vector<pair<string, Json::Value>> attributes = {
diff --git a/libsolidity/ast/ASTJsonConverter.h b/libsolidity/ast/ASTJsonConverter.h
index b23978e10..904749004 100644
--- a/libsolidity/ast/ASTJsonConverter.h
+++ b/libsolidity/ast/ASTJsonConverter.h
@@ -88,6 +88,7 @@ public:
 	bool visit(ModifierDefinition const& _node) override;
 	bool visit(ModifierInvocation const& _node) override;
 	bool visit(EventDefinition const& _node) override;
+	bool visit(ErrorDefinition const& _node) override;
 	bool visit(ElementaryTypeName const& _node) override;
 	bool visit(UserDefinedTypeName const& _node) override;
 	bool visit(FunctionTypeName const& _node) override;
diff --git a/libsolidity/ast/ASTJsonImporter.cpp b/libsolidity/ast/ASTJsonImporter.cpp
index fef38e5d3..cb58885f7 100644
--- a/libsolidity/ast/ASTJsonImporter.cpp
+++ b/libsolidity/ast/ASTJsonImporter.cpp
@@ -151,6 +151,8 @@ ASTPointer<ASTNode> ASTJsonImporter::convertJsonToASTNode(Json::Value const& _js
 		return createModifierInvocation(_json);
 	if (nodeType == "EventDefinition")
 		return createEventDefinition(_json);
+	if (nodeType == "ErrorDefinition")
+		return createErrorDefinition(_json);
 	if (nodeType == "ElementaryTypeName")
 		return createElementaryTypeName(_json);
 	if (nodeType == "UserDefinedTypeName")
@@ -539,6 +541,17 @@ ASTPointer<EventDefinition> ASTJsonImporter::createEventDefinition(Json::Value c
 	);
 }
 
+ASTPointer<ErrorDefinition> ASTJsonImporter::createErrorDefinition(Json::Value const&  _node)
+{
+	return createASTNode<ErrorDefinition>(
+		_node,
+		memberAsASTString(_node, "name"),
+		createNameSourceLocation(_node),
+		_node["documentation"].isNull() ? nullptr : createDocumentation(member(_node, "documentation")),
+		createParameterList(member(_node, "parameters"))
+	);
+}
+
 ASTPointer<ElementaryTypeName> ASTJsonImporter::createElementaryTypeName(Json::Value const& _node)
 {
 	unsigned short firstNum;
diff --git a/libsolidity/ast/ASTJsonImporter.h b/libsolidity/ast/ASTJsonImporter.h
index c4f8c6cc9..3ef6db384 100644
--- a/libsolidity/ast/ASTJsonImporter.h
+++ b/libsolidity/ast/ASTJsonImporter.h
@@ -88,6 +88,7 @@ private:
 	ASTPointer<ModifierDefinition> createModifierDefinition(Json::Value const& _node);
 	ASTPointer<ModifierInvocation> createModifierInvocation(Json::Value const& _node);
 	ASTPointer<EventDefinition> createEventDefinition(Json::Value const& _node);
+	ASTPointer<ErrorDefinition> createErrorDefinition(Json::Value const& _node);
 	ASTPointer<ElementaryTypeName> createElementaryTypeName(Json::Value const& _node);
 	ASTPointer<UserDefinedTypeName> createUserDefinedTypeName(Json::Value const& _node);
 	ASTPointer<FunctionTypeName> createFunctionTypeName(Json::Value const& _node);
diff --git a/libsolidity/ast/ASTUtils.h b/libsolidity/ast/ASTUtils.h
index 13626a0cf..44caa24dd 100644
--- a/libsolidity/ast/ASTUtils.h
+++ b/libsolidity/ast/ASTUtils.h
@@ -22,6 +22,12 @@ namespace solidity::frontend
 {
 
 class VariableDeclaration;
+class Declaration;
+class Expression;
+
+/// @returns the declaration referenced from the expression which has to be MemberAccess
+/// or Identifier. Returns nullptr otherwise.
+Declaration const* referencedDeclaration(Expression const& _expression);
 
 /// Find the topmost referenced constant variable declaration when the given variable
 /// declaration value is an identifier. Works only for constant variable declarations.
diff --git a/libsolidity/ast/ASTVisitor.h b/libsolidity/ast/ASTVisitor.h
index 9ad652b06..4acb47273 100644
--- a/libsolidity/ast/ASTVisitor.h
+++ b/libsolidity/ast/ASTVisitor.h
@@ -71,6 +71,7 @@ public:
 	virtual bool visit(ModifierDefinition& _node) { return visitNode(_node); }
 	virtual bool visit(ModifierInvocation& _node) { return visitNode(_node); }
 	virtual bool visit(EventDefinition& _node) { return visitNode(_node); }
+	virtual bool visit(ErrorDefinition& _node) { return visitNode(_node); }
 	virtual bool visit(ElementaryTypeName& _node) { return visitNode(_node); }
 	virtual bool visit(UserDefinedTypeName& _node) { return visitNode(_node); }
 	virtual bool visit(FunctionTypeName& _node) { return visitNode(_node); }
@@ -124,6 +125,7 @@ public:
 	virtual void endVisit(ModifierDefinition& _node) { endVisitNode(_node); }
 	virtual void endVisit(ModifierInvocation& _node) { endVisitNode(_node); }
 	virtual void endVisit(EventDefinition& _node) { endVisitNode(_node); }
+	virtual void endVisit(ErrorDefinition& _node) { endVisitNode(_node); }
 	virtual void endVisit(ElementaryTypeName& _node) { endVisitNode(_node); }
 	virtual void endVisit(UserDefinedTypeName& _node) { endVisitNode(_node); }
 	virtual void endVisit(FunctionTypeName& _node) { endVisitNode(_node); }
@@ -199,6 +201,7 @@ public:
 	virtual bool visit(ModifierDefinition const& _node) { return visitNode(_node); }
 	virtual bool visit(ModifierInvocation const& _node) { return visitNode(_node); }
 	virtual bool visit(EventDefinition const& _node) { return visitNode(_node); }
+	virtual bool visit(ErrorDefinition const& _node) { return visitNode(_node); }
 	virtual bool visit(ElementaryTypeName const& _node) { return visitNode(_node); }
 	virtual bool visit(UserDefinedTypeName const& _node) { return visitNode(_node); }
 	virtual bool visit(FunctionTypeName const& _node) { return visitNode(_node); }
@@ -252,6 +255,7 @@ public:
 	virtual void endVisit(ModifierDefinition const& _node) { endVisitNode(_node); }
 	virtual void endVisit(ModifierInvocation const& _node) { endVisitNode(_node); }
 	virtual void endVisit(EventDefinition const& _node) { endVisitNode(_node); }
+	virtual void endVisit(ErrorDefinition const& _node) { endVisitNode(_node); }
 	virtual void endVisit(ElementaryTypeName const& _node) { endVisitNode(_node); }
 	virtual void endVisit(UserDefinedTypeName const& _node) { endVisitNode(_node); }
 	virtual void endVisit(FunctionTypeName const& _node) { endVisitNode(_node); }
diff --git a/libsolidity/ast/AST_accept.h b/libsolidity/ast/AST_accept.h
index afd9838a1..678a9ef7e 100644
--- a/libsolidity/ast/AST_accept.h
+++ b/libsolidity/ast/AST_accept.h
@@ -366,6 +366,28 @@ void EventDefinition::accept(ASTConstVisitor& _visitor) const
 	_visitor.endVisit(*this);
 }
 
+void ErrorDefinition::accept(ASTVisitor& _visitor)
+{
+	if (_visitor.visit(*this))
+	{
+		if (m_documentation)
+			m_documentation->accept(_visitor);
+		m_parameters->accept(_visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
+void ErrorDefinition::accept(ASTConstVisitor& _visitor) const
+{
+	if (_visitor.visit(*this))
+	{
+		if (m_documentation)
+			m_documentation->accept(_visitor);
+		m_parameters->accept(_visitor);
+	}
+	_visitor.endVisit(*this);
+}
+
 void ElementaryTypeName::accept(ASTVisitor& _visitor)
 {
 	_visitor.visit(*this);
diff --git a/libsolidity/ast/TypeProvider.cpp b/libsolidity/ast/TypeProvider.cpp
index c30c6fd75..cc810708e 100644
--- a/libsolidity/ast/TypeProvider.cpp
+++ b/libsolidity/ast/TypeProvider.cpp
@@ -431,6 +431,11 @@ FunctionType const* TypeProvider::function(EventDefinition const& _def)
 	return createAndGet<FunctionType>(_def);
 }
 
+FunctionType const* TypeProvider::function(ErrorDefinition const& _def)
+{
+	return createAndGet<FunctionType>(_def);
+}
+
 FunctionType const* TypeProvider::function(FunctionTypeName const& _typeName)
 {
 	return createAndGet<FunctionType>(_typeName);
diff --git a/libsolidity/ast/TypeProvider.h b/libsolidity/ast/TypeProvider.h
index 007e56788..6cdfdadc6 100644
--- a/libsolidity/ast/TypeProvider.h
+++ b/libsolidity/ast/TypeProvider.h
@@ -139,6 +139,8 @@ public:
 	/// @returns the function type of an event.
 	static FunctionType const* function(EventDefinition const& _event);
 
+	static FunctionType const* function(ErrorDefinition const& _error);
+
 	/// @returns the type of a function type name.
 	static FunctionType const* function(FunctionTypeName const& _typeName);
 
diff --git a/libsolidity/ast/Types.cpp b/libsolidity/ast/Types.cpp
index 1accdeb81..b098d6f3f 100644
--- a/libsolidity/ast/Types.cpp
+++ b/libsolidity/ast/Types.cpp
@@ -2736,13 +2736,36 @@ FunctionType::FunctionType(EventDefinition const& _event):
 	}
 
 	solAssert(
-			m_parameterNames.size() == m_parameterTypes.size(),
-			"Parameter names list must match parameter types list!"
-			);
+		m_parameterNames.size() == m_parameterTypes.size(),
+		"Parameter names list must match parameter types list!"
+	);
 	solAssert(
-			m_returnParameterNames.size() == m_returnParameterTypes.size(),
-			"Return parameter names list must match return parameter types list!"
-			);
+		m_returnParameterNames.size() == 0 &&
+		m_returnParameterTypes.size() == 0,
+		""
+	);
+}
+
+FunctionType::FunctionType(ErrorDefinition const& _error):
+	m_kind(Kind::Error),
+	m_stateMutability(StateMutability::Pure),
+	m_declaration(&_error)
+{
+	for (ASTPointer<VariableDeclaration> const& var: _error.parameters())
+	{
+		m_parameterNames.push_back(var->name());
+		m_parameterTypes.push_back(var->annotation().type);
+	}
+
+	solAssert(
+		m_parameterNames.size() == m_parameterTypes.size(),
+		"Parameter names list must match parameter types list!"
+	);
+	solAssert(
+		m_returnParameterNames.size() == 0 &&
+		m_returnParameterTypes.size() == 0,
+		""
+	);
 }
 
 FunctionType::FunctionType(FunctionTypeName const& _typeName):
@@ -2870,6 +2893,7 @@ string FunctionType::richIdentifier() const
 	case Kind::RIPEMD160: id += "ripemd160"; break;
 	case Kind::GasLeft: id += "gasleft"; break;
 	case Kind::Event: id += "event"; break;
+	case Kind::Error: id += "error"; break;
 	case Kind::SetGas: id += "setgas"; break;
 	case Kind::SetValue: id += "setvalue"; break;
 	case Kind::BlockHash: id += "blockhash"; break;
@@ -3107,7 +3131,7 @@ FunctionTypePointer FunctionType::interfaceFunctionType() const
 	// Note that m_declaration might also be a state variable!
 	solAssert(m_declaration, "Declaration needed to determine interface function type.");
 	bool isLibraryFunction = false;
-	if (kind() != Kind::Event)
+	if (kind() != Kind::Event && kind() != Kind::Error)
 		if (auto const* contract = dynamic_cast<ContractDefinition const*>(m_declaration->scope()))
 			isLibraryFunction = contract->isLibrary();
 
@@ -3230,6 +3254,8 @@ MemberList::MemberMap FunctionType::nativeMembers(ASTNode const* _scope) const
 		}
 		return {};
 	}
+	case Kind::Error:
+		return {{"selector", TypeProvider::fixedBytes(4)}};
 	default:
 		return MemberList::MemberMap();
 	}
@@ -3396,15 +3422,16 @@ string FunctionType::externalSignature() const
 	case Kind::External:
 	case Kind::DelegateCall:
 	case Kind::Event:
+	case Kind::Error:
 	case Kind::Declaration:
 		break;
 	default:
 		solAssert(false, "Invalid function type for requesting external signature.");
 	}
 
-	// "inLibrary" is only relevant if this is not an event.
+	// "inLibrary" is only relevant if this is neither an event nor an error.
 	bool inLibrary = false;
-	if (kind() != Kind::Event)
+	if (kind() != Kind::Event && kind() != Kind::Error)
 		if (auto const* contract = dynamic_cast<ContractDefinition const*>(m_declaration->scope()))
 			inLibrary = contract->isLibrary();
 
diff --git a/libsolidity/ast/Types.h b/libsolidity/ast/Types.h
index 469779c8b..7bcd3d44c 100644
--- a/libsolidity/ast/Types.h
+++ b/libsolidity/ast/Types.h
@@ -1151,6 +1151,7 @@ public:
 		SHA256, ///< CALL to special contract for sha256
 		RIPEMD160, ///< CALL to special contract for ripemd160
 		Event, ///< syntactic sugar for LOG*
+		Error, ///< creating an error instance in revert or require
 		SetGas, ///< modify the default gas value for the function call
 		SetValue, ///< modify the default value transfer for the function call
 		BlockHash, ///< BLOCKHASH
@@ -1182,6 +1183,7 @@ public:
 	explicit FunctionType(VariableDeclaration const& _varDecl);
 	/// Creates the function type of an event.
 	explicit FunctionType(EventDefinition const& _event);
+	explicit FunctionType(ErrorDefinition const& _error);
 	/// Creates the type of a function type name.
 	explicit FunctionType(FunctionTypeName const& _typeName);
 	/// Function type constructor to be used for a plain type (not derived from a declaration).
diff --git a/libsolidity/codegen/ExpressionCompiler.cpp b/libsolidity/codegen/ExpressionCompiler.cpp
index 7e82da6c4..fb88e0dd2 100644
--- a/libsolidity/codegen/ExpressionCompiler.cpp
+++ b/libsolidity/codegen/ExpressionCompiler.cpp
@@ -912,6 +912,10 @@ bool ExpressionCompiler::visit(FunctionCall const& _functionCall)
 			m_context << logInstruction(numIndexed);
 			break;
 		}
+		case FunctionType::Kind::Error:
+		{
+			solAssert(false, "");
+		}
 		case FunctionType::Kind::BlockHash:
 		{
 			acceptAndConvert(*arguments[0], *function.parameterTypes()[0], true);
@@ -1420,6 +1424,11 @@ bool ExpressionCompiler::visit(MemberAccess const& _memberAccess)
 							solAssert(false, "event not found");
 						// no-op, because the parent node will do the job
 						break;
+					case FunctionType::Kind::Error:
+						if (!dynamic_cast<ErrorDefinition const*>(_memberAccess.annotation().referencedDeclaration))
+							solAssert(false, "error not found");
+						// no-op, because the parent node will do the job
+						break;
 					case FunctionType::Kind::DelegateCall:
 						_memberAccess.expression().accept(*this);
 						m_context << funType->externalIdentifier();
@@ -2091,6 +2100,10 @@ void ExpressionCompiler::endVisit(Identifier const& _identifier)
 	{
 		// no-op
 	}
+	else if (dynamic_cast<ErrorDefinition const*>(declaration))
+	{
+		// no-op
+	}
 	else if (dynamic_cast<EnumDefinition const*>(declaration))
 	{
 		// no-op
diff --git a/libsolidity/codegen/ir/IRGeneratorForStatements.cpp b/libsolidity/codegen/ir/IRGeneratorForStatements.cpp
index 2578c0fb5..dddf8b80c 100644
--- a/libsolidity/codegen/ir/IRGeneratorForStatements.cpp
+++ b/libsolidity/codegen/ir/IRGeneratorForStatements.cpp
@@ -1043,6 +1043,10 @@ void IRGeneratorForStatements::endVisit(FunctionCall const& _functionCall)
 		m_code << templ.render();
 		break;
 	}
+	case FunctionType::Kind::Error:
+	{
+		solAssert(false, "");
+	}
 	case FunctionType::Kind::Assert:
 	case FunctionType::Kind::Require:
 	{
@@ -1710,13 +1714,18 @@ void IRGeneratorForStatements::endVisit(MemberAccess const& _memberAccess)
 				define(IRVariable{_memberAccess}, IRVariable(_memberAccess.expression()).part("functionSelector"));
 			else if (
 				functionType.kind() == FunctionType::Kind::Declaration ||
+				functionType.kind() == FunctionType::Kind::Error ||
 				// In some situations, internal function types also provide the "selector" member.
 				// See Types.cpp for details.
 				functionType.kind() == FunctionType::Kind::Internal
 			)
 			{
 				solAssert(functionType.hasDeclaration(), "");
-				solAssert(functionType.declaration().isPartOfExternalInterface(), "");
+				solAssert(
+					functionType.kind() == FunctionType::Kind::Error ||
+					functionType.declaration().isPartOfExternalInterface(),
+					""
+				);
 				define(IRVariable{_memberAccess}) << formatNumber(functionType.externalIdentifier() << 224) << "\n";
 			}
 			else
@@ -1981,6 +1990,13 @@ void IRGeneratorForStatements::endVisit(MemberAccess const& _memberAccess)
 					);
 						// the call will do the resolving
 					break;
+				case FunctionType::Kind::Error:
+					solAssert(
+						dynamic_cast<ErrorDefinition const*>(_memberAccess.annotation().referencedDeclaration),
+						"Error not found"
+					);
+						// the call will do the resolving
+					break;
 				case FunctionType::Kind::DelegateCall:
 					define(IRVariable(_memberAccess).part("address"), _memberAccess.expression());
 					define(IRVariable(_memberAccess).part("functionSelector")) << formatNumber(memberFunctionType->externalIdentifier()) << "\n";
@@ -2310,6 +2326,10 @@ void IRGeneratorForStatements::endVisit(Identifier const& _identifier)
 	{
 		// no-op
 	}
+	else if (dynamic_cast<ErrorDefinition const*>(declaration))
+	{
+		// no-op
+	}
 	else if (dynamic_cast<EnumDefinition const*>(declaration))
 	{
 		// no-op
diff --git a/libsolidity/formal/SMTEncoder.cpp b/libsolidity/formal/SMTEncoder.cpp
index 9cb9472d1..82498745d 100644
--- a/libsolidity/formal/SMTEncoder.cpp
+++ b/libsolidity/formal/SMTEncoder.cpp
@@ -699,6 +699,7 @@ void SMTEncoder::endVisit(FunctionCall const& _funCall)
 		arrayPop(_funCall);
 		break;
 	case FunctionType::Kind::Event:
+	case FunctionType::Kind::Error:
 		// This can be safely ignored.
 		break;
 	case FunctionType::Kind::ObjectCreation:
diff --git a/libsolidity/parsing/Parser.cpp b/libsolidity/parsing/Parser.cpp
index 6dc0dfa6b..e05134310 100644
--- a/libsolidity/parsing/Parser.cpp
+++ b/libsolidity/parsing/Parser.cpp
@@ -112,8 +112,16 @@ ASTPointer<SourceUnit> Parser::parse(shared_ptr<Scanner> const& _scanner)
 				nodes.push_back(parseFunctionDefinition(true));
 				break;
 			default:
+				if (
+					// Workaround because `error` is not a keyword.
+					m_scanner->currentToken() == Token::Identifier &&
+					currentLiteral() == "error" &&
+					m_scanner->peekNextToken() == Token::Identifier &&
+					m_scanner->peekNextNextToken() == Token::LParen
+				)
+					nodes.push_back(parseErrorDefinition());
 				// Constant variable.
-				if (variableDeclarationStart() && m_scanner->peekNextToken() != Token::EOS)
+				else if (variableDeclarationStart() && m_scanner->peekNextToken() != Token::EOS)
 				{
 					VarDeclParserOptions options;
 					options.kind = VarDeclKind::FileLevel;
@@ -351,6 +359,14 @@ ASTPointer<ContractDefinition> Parser::parseContractDefinition()
 				subNodes.push_back(parseStructDefinition());
 			else if (currentTokenValue == Token::Enum)
 				subNodes.push_back(parseEnumDefinition());
+			else if (
+				// Workaround because `error` is not a keyword.
+				currentTokenValue == Token::Identifier &&
+				currentLiteral() == "error" &&
+				m_scanner->peekNextToken() == Token::Identifier &&
+				m_scanner->peekNextNextToken() == Token::LParen
+			)
+				subNodes.push_back(parseErrorDefinition());
 			else if (variableDeclarationStart())
 			{
 				VarDeclParserOptions options;
@@ -917,6 +933,21 @@ ASTPointer<EventDefinition> Parser::parseEventDefinition()
 	return nodeFactory.createNode<EventDefinition>(name, nameLocation, documentation, parameters, anonymous);
 }
 
+ASTPointer<ErrorDefinition> Parser::parseErrorDefinition()
+{
+	RecursionGuard recursionGuard(*this);
+	ASTNodeFactory nodeFactory(*this);
+	ASTPointer<StructuredDocumentation> documentation = parseStructuredDocumentation();
+
+	solAssert(*expectIdentifierToken() == "error", "");
+	auto&& [name, nameLocation] = expectIdentifierWithLocation();
+
+	ASTPointer<ParameterList> parameters = parseParameterList({});
+	nodeFactory.markEndPosition();
+	expectToken(Token::Semicolon);
+	return nodeFactory.createNode<ErrorDefinition>(name, move(nameLocation), documentation, parameters);
+}
+
 ASTPointer<UsingForDirective> Parser::parseUsingDirective()
 {
 	RecursionGuard recursionGuard(*this);
diff --git a/libsolidity/parsing/Parser.h b/libsolidity/parsing/Parser.h
index 77d6cd17c..dd9125e5b 100644
--- a/libsolidity/parsing/Parser.h
+++ b/libsolidity/parsing/Parser.h
@@ -102,6 +102,7 @@ private:
 	);
 	ASTPointer<ModifierDefinition> parseModifierDefinition();
 	ASTPointer<EventDefinition> parseEventDefinition();
+	ASTPointer<ErrorDefinition> parseErrorDefinition();
 	ASTPointer<UsingForDirective> parseUsingDirective();
 	ASTPointer<ModifierInvocation> parseModifierInvocation();
 	ASTPointer<Identifier> parseIdentifier();
diff --git a/test/libsolidity/semanticTests/error/selector.sol b/test/libsolidity/semanticTests/error/selector.sol
new file mode 100644
index 000000000..fab71f443
--- /dev/null
+++ b/test/libsolidity/semanticTests/error/selector.sol
@@ -0,0 +1,20 @@
+library L {
+    error E();
+}
+library S {
+    error E(uint);
+}
+library T {
+    error E();
+}
+contract C {
+    function f() public pure returns (bytes4, bytes4) {
+        assert(L.E.selector == T.E.selector);
+        assert(L.E.selector != S.E.selector);
+        return (L.E.selector, S.E.selector);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0x92bbf6e800000000000000000000000000000000000000000000000000000000, 0x2ff06700000000000000000000000000000000000000000000000000000000
diff --git a/test/libsolidity/syntaxTests/errors/abi_decode_error.sol b/test/libsolidity/syntaxTests/errors/abi_decode_error.sol
new file mode 100644
index 000000000..72b661b6f
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/abi_decode_error.sol
@@ -0,0 +1,9 @@
+error E(uint);
+contract C {
+    function f() public pure returns (bytes memory) {
+        return abi.decode(msg.data, (E));
+    }
+}
+// ----
+// TypeError 1039: (119-120): Argument has to be a type name.
+// TypeError 5132: (90-122): Different number of arguments in return statement than in returns declaration.
diff --git a/test/libsolidity/syntaxTests/errors/abi_encode_error.sol b/test/libsolidity/syntaxTests/errors/abi_encode_error.sol
new file mode 100644
index 000000000..55b4d73df
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/abi_encode_error.sol
@@ -0,0 +1,8 @@
+error E(uint);
+contract C {
+    function f() public pure returns (bytes memory) {
+        return abi.encode(E);
+    }
+}
+// ----
+// TypeError 2056: (108-109): This type cannot be encoded.
diff --git a/test/libsolidity/syntaxTests/errors/abi_encode_error_instance.sol b/test/libsolidity/syntaxTests/errors/abi_encode_error_instance.sol
new file mode 100644
index 000000000..fbc8b97f1
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/abi_encode_error_instance.sol
@@ -0,0 +1,8 @@
+error E(uint);
+contract C {
+    function f() public pure returns (bytes memory) {
+        return abi.encode(E(2));
+    }
+}
+// ----
+// TypeError 2056: (108-112): This type cannot be encoded.
diff --git a/test/libsolidity/syntaxTests/errors/all_ones_signature.sol b/test/libsolidity/syntaxTests/errors/all_ones_signature.sol
new file mode 100644
index 000000000..4cadf624d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/all_ones_signature.sol
@@ -0,0 +1,8 @@
+error test266151307();
+contract C {
+    error test266151307();
+}
+// ----
+// Warning 2519: (40-62): This declaration shadows an existing declaration.
+// SyntaxError 2855: (0-22): The selector 0xffffffff is reserved. Please rename the error to avoid the collision.
+// SyntaxError 2855: (40-62): The selector 0xffffffff is reserved. Please rename the error to avoid the collision.
diff --git a/test/libsolidity/syntaxTests/errors/anonymous.sol b/test/libsolidity/syntaxTests/errors/anonymous.sol
new file mode 100644
index 000000000..5cbf95f2d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/anonymous.sol
@@ -0,0 +1,3 @@
+error E() anonymous;
+// ----
+// ParserError 2314: (10-19): Expected ';' but got 'anonymous'
diff --git a/test/libsolidity/syntaxTests/errors/assert_with_cond_and_error.sol b/test/libsolidity/syntaxTests/errors/assert_with_cond_and_error.sol
new file mode 100644
index 000000000..45ac6bfa1
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/assert_with_cond_and_error.sol
@@ -0,0 +1,6 @@
+error E();
+function f(bool x) pure {
+    assert(x, E());
+}
+// ----
+// TypeError 6160: (41-55): Wrong argument count for function call: 2 arguments given but expected 1.
diff --git a/test/libsolidity/syntaxTests/errors/assert_with_error.sol b/test/libsolidity/syntaxTests/errors/assert_with_error.sol
new file mode 100644
index 000000000..90f79b40e
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/assert_with_error.sol
@@ -0,0 +1,6 @@
+error E();
+function f() pure {
+    assert(E());
+}
+// ----
+// TypeError 9553: (42-45): Invalid type for argument in function call. Invalid implicit conversion from tuple() to bool requested.
diff --git a/test/libsolidity/syntaxTests/errors/basic.sol b/test/libsolidity/syntaxTests/errors/basic.sol
new file mode 100644
index 000000000..75ac3cf5e
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/basic.sol
@@ -0,0 +1,6 @@
+contract C {
+    error MyError();
+    error MyError2(uint x);
+    error MyError3(uint x, bytes);
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/clash_function_error.sol b/test/libsolidity/syntaxTests/errors/clash_function_error.sol
new file mode 100644
index 000000000..0e3a97f7f
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/clash_function_error.sol
@@ -0,0 +1,4 @@
+function Err() pure {}
+error Err();
+// ----
+// DeclarationError 2333: (23-35): Identifier already declared.
diff --git a/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance.sol b/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance.sol
new file mode 100644
index 000000000..3b71fb4d6
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance.sol
@@ -0,0 +1,4 @@
+contract A { function Err() public pure {} }
+contract B is A { error Err(); }
+// ----
+// DeclarationError 9097: (63-75): Identifier already declared.
diff --git a/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance_2.sol b/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance_2.sol
new file mode 100644
index 000000000..bfe4e823f
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/clash_function_error_inheritance_2.sol
@@ -0,0 +1,5 @@
+contract A { function Err() public pure {} }
+contract B { error Err(); }
+contract C is A, B {}
+// ----
+// DeclarationError 9097: (58-70): Identifier already declared.
diff --git a/test/libsolidity/syntaxTests/errors/error_address_payable.sol b/test/libsolidity/syntaxTests/errors/error_address_payable.sol
new file mode 100644
index 000000000..3e983e374
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_address_payable.sol
@@ -0,0 +1,2 @@
+error E(address payable x);
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/error_as_function_param.sol b/test/libsolidity/syntaxTests/errors/error_as_function_param.sol
new file mode 100644
index 000000000..3741687f6
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_as_function_param.sol
@@ -0,0 +1,4 @@
+error E(uint);
+function f(E x) pure returns (uint) {}
+// ----
+// TypeError 5172: (26-27): Name has to refer to a struct, enum or contract.
diff --git a/test/libsolidity/syntaxTests/errors/error_in_interface.sol b/test/libsolidity/syntaxTests/errors/error_in_interface.sol
new file mode 100644
index 000000000..3f0f89b61
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_in_interface.sol
@@ -0,0 +1,4 @@
+interface C {
+    error E(uint);
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/error_in_library.sol b/test/libsolidity/syntaxTests/errors/error_in_library.sol
new file mode 100644
index 000000000..1be9718b0
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_in_library.sol
@@ -0,0 +1,4 @@
+library L {
+    error E(uint);
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/error_location_memory.sol b/test/libsolidity/syntaxTests/errors/error_location_memory.sol
new file mode 100644
index 000000000..c40d8dfc2
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_location_memory.sol
@@ -0,0 +1,3 @@
+error E(uint[] memory);
+// ----
+// ParserError 2314: (15-21): Expected ',' but got 'memory'
diff --git a/test/libsolidity/syntaxTests/errors/error_location_specifier.sol b/test/libsolidity/syntaxTests/errors/error_location_specifier.sol
new file mode 100644
index 000000000..5f5bdc2bb
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_location_specifier.sol
@@ -0,0 +1,3 @@
+error E(uint[] calldata);
+// ----
+// ParserError 2314: (15-23): Expected ',' but got 'calldata'
diff --git a/test/libsolidity/syntaxTests/errors/error_reserved_name.sol b/test/libsolidity/syntaxTests/errors/error_reserved_name.sol
new file mode 100644
index 000000000..fc89efc18
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/error_reserved_name.sol
@@ -0,0 +1,3 @@
+error Error(uint);
+// ----
+// SyntaxError 1855: (0-18): The built-in errors "Error" and "Panic" cannot be re-defined.
diff --git a/test/libsolidity/syntaxTests/errors/file_level.sol b/test/libsolidity/syntaxTests/errors/file_level.sol
new file mode 100644
index 000000000..07afb90cc
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/file_level.sol
@@ -0,0 +1,6 @@
+error MyError();
+error MyError2(uint x);
+contract C {
+    error MyError3(uint x, bytes);
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/hash_collision.sol b/test/libsolidity/syntaxTests/errors/hash_collision.sol
new file mode 100644
index 000000000..37b95c822
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/hash_collision.sol
@@ -0,0 +1,6 @@
+contract test {
+    error gsf();
+    error tgeo();
+}
+// ----
+// TypeError 4883: (0-52): Error signature hash collision for tgeo()
diff --git a/test/libsolidity/syntaxTests/errors/indexed_error.sol b/test/libsolidity/syntaxTests/errors/indexed_error.sol
new file mode 100644
index 000000000..2efa3d70d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/indexed_error.sol
@@ -0,0 +1,3 @@
+error E(uint indexed);
+// ----
+// ParserError 2314: (13-20): Expected ',' but got 'indexed'
diff --git a/test/libsolidity/syntaxTests/errors/no_mappings.sol b/test/libsolidity/syntaxTests/errors/no_mappings.sol
new file mode 100644
index 000000000..9a8c9e509
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/no_mappings.sol
@@ -0,0 +1,9 @@
+error MyError(mapping(uint => uint));
+contract C {
+    error MyError2(mapping(uint => uint));
+}
+// ----
+// TypeError 3448: (14-35): Type containing a (nested) mapping is not allowed as error parameter type.
+// TypeError 3417: (14-35): Internal or recursive type is not allowed as error parameter type.
+// TypeError 3448: (70-91): Type containing a (nested) mapping is not allowed as error parameter type.
+// TypeError 3417: (70-91): Internal or recursive type is not allowed as error parameter type.
diff --git a/test/libsolidity/syntaxTests/errors/no_overloading.sol b/test/libsolidity/syntaxTests/errors/no_overloading.sol
new file mode 100644
index 000000000..53ba01743
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/no_overloading.sol
@@ -0,0 +1,4 @@
+error Err(uint);
+error Err(bytes32);
+// ----
+// DeclarationError 2333: (17-36): Identifier already declared.
diff --git a/test/libsolidity/syntaxTests/errors/no_overloading_inheritance.sol b/test/libsolidity/syntaxTests/errors/no_overloading_inheritance.sol
new file mode 100644
index 000000000..fac0da4bc
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/no_overloading_inheritance.sol
@@ -0,0 +1,8 @@
+contract A {
+    error Err(uint);
+}
+contract B is A {
+    error Err(bytes32);
+}
+// ----
+// DeclarationError 9097: (58-77): Identifier already declared.
diff --git a/test/libsolidity/syntaxTests/errors/no_structs_in_abiv1.sol b/test/libsolidity/syntaxTests/errors/no_structs_in_abiv1.sol
new file mode 100644
index 000000000..8727a2852
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/no_structs_in_abiv1.sol
@@ -0,0 +1,7 @@
+pragma abicoder v1;
+struct S {uint a;}
+contract C {
+    error MyError(S);
+}
+// ----
+// TypeError 3061: (70-71): This type is only supported in ABI coder v2. Use "pragma abicoder v2;" to enable the feature.
diff --git a/test/libsolidity/syntaxTests/errors/panic_reserved_name.sol b/test/libsolidity/syntaxTests/errors/panic_reserved_name.sol
new file mode 100644
index 000000000..c6f3dd1af
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/panic_reserved_name.sol
@@ -0,0 +1,3 @@
+error Panic(bytes2);
+// ----
+// SyntaxError 1855: (0-20): The built-in errors "Error" and "Panic" cannot be re-defined.
diff --git a/test/libsolidity/syntaxTests/errors/selector.sol b/test/libsolidity/syntaxTests/errors/selector.sol
new file mode 100644
index 000000000..6d8826517
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/selector.sol
@@ -0,0 +1,6 @@
+error E();
+
+contract C {
+    bytes4 t = E.selector;
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/selector_on_instance.sol b/test/libsolidity/syntaxTests/errors/selector_on_instance.sol
new file mode 100644
index 000000000..3eac5984f
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/selector_on_instance.sol
@@ -0,0 +1,7 @@
+error E();
+
+contract C {
+    bytes4 t = E().selector;
+}
+// ----
+// TypeError 9582: (40-52): Member "selector" not found or not visible after argument-dependent lookup in tuple().
diff --git a/test/libsolidity/syntaxTests/errors/struct_named_error.sol b/test/libsolidity/syntaxTests/errors/struct_named_error.sol
new file mode 100644
index 000000000..0ee82ead6
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/struct_named_error.sol
@@ -0,0 +1,6 @@
+// Test that the parser workaround is not breaking.
+struct error {uint a;}
+contract C {
+    error x;
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/using.sol b/test/libsolidity/syntaxTests/errors/using.sol
new file mode 100644
index 000000000..388fa52ab
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/using.sol
@@ -0,0 +1,12 @@
+error E(uint);
+library L {
+    function f(uint) internal {}
+}
+contract C {
+    using L for *;
+    function f() public pure {
+        E.f();
+    }
+}
+// ----
+// TypeError 9582: (133-136): Member "f" not found or not visible after argument-dependent lookup in function (uint256) pure.
diff --git a/test/libsolidity/syntaxTests/errors/using_2.sol b/test/libsolidity/syntaxTests/errors/using_2.sol
new file mode 100644
index 000000000..2f60f3d04
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/using_2.sol
@@ -0,0 +1,12 @@
+error E(uint);
+library L {
+    function f(uint) internal {}
+}
+contract C {
+    using L for E;
+    function f() public pure {
+        E.f();
+    }
+}
+// ----
+// TypeError 5172: (91-92): Name has to refer to a struct, enum or contract.
diff --git a/test/libsolidity/syntaxTests/errors/using_structs.sol b/test/libsolidity/syntaxTests/errors/using_structs.sol
new file mode 100644
index 000000000..282fce35d
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/using_structs.sol
@@ -0,0 +1,6 @@
+struct S {uint a;}
+contract C {
+    error MyError(S);
+    error MyError2(S t);
+}
+// ----
diff --git a/test/libsolidity/syntaxTests/errors/weird1.sol b/test/libsolidity/syntaxTests/errors/weird1.sol
new file mode 100644
index 000000000..b57f34c22
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/weird1.sol
@@ -0,0 +1,7 @@
+error E();
+
+contract C {
+    function() internal pure x = E;
+}
+// ----
+// TypeError 7407: (58-59): Type function () pure is not implicitly convertible to expected type function () pure. Special functions can not be converted to function types.
diff --git a/test/libsolidity/syntaxTests/errors/weird3.sol b/test/libsolidity/syntaxTests/errors/weird3.sol
new file mode 100644
index 000000000..c81266969
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/weird3.sol
@@ -0,0 +1,7 @@
+error E();
+
+contract C {
+    E x;
+}
+// ----
+// TypeError 5172: (29-30): Name has to refer to a struct, enum or contract.
diff --git a/test/libsolidity/syntaxTests/errors/weird4.sol b/test/libsolidity/syntaxTests/errors/weird4.sol
new file mode 100644
index 000000000..be1ef90b6
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/weird4.sol
@@ -0,0 +1,9 @@
+error E();
+
+contract C {
+    function f() public pure {
+        E x;
+    }
+}
+// ----
+// TypeError 5172: (64-65): Name has to refer to a struct, enum or contract.
diff --git a/test/libsolidity/syntaxTests/errors/zero_signature.sol b/test/libsolidity/syntaxTests/errors/zero_signature.sol
new file mode 100644
index 000000000..06a212d76
--- /dev/null
+++ b/test/libsolidity/syntaxTests/errors/zero_signature.sol
@@ -0,0 +1,8 @@
+error buyAndFree22457070633(uint256);
+contract C {
+    error buyAndFree22457070633(uint256);
+}
+// ----
+// Warning 2519: (55-92): This declaration shadows an existing declaration.
+// SyntaxError 2855: (0-37): The selector 0x00000000 is reserved. Please rename the error to avoid the collision.
+// SyntaxError 2855: (55-92): The selector 0x00000000 is reserved. Please rename the error to avoid the collision.