diff --git a/Cargo.lock b/Cargo.lock
index c679d5d78..32be4e46d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -86,7 +86,7 @@ dependencies = [
 [[package]]
 name = "amcl"
 version = "0.2.0"
-source = "git+https://github.com/sigp/milagro_bls?branch=eth2.0-v0.10#9f946ca454ad1da4cd1344bd632b7306704bc904"
+source = "git+https://github.com/sigp/milagro_bls?branch=eth2.0-v0.10#38c6c33925b24c9319a1febfb621ff9bbf6d49f7"
 dependencies = [
  "hex 0.3.2",
  "lazy_static",
@@ -144,7 +144,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0d0864d84b8e07b145449be9a8537db86bf9de5ce03b913214694643b4743502"
 dependencies = [
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
 ]
 
 [[package]]
@@ -172,9 +172,9 @@ checksum = "f8aac770f1885fd7e387acedd76065302551364496e46b3dd00860b2f8359b9d"
 
 [[package]]
 name = "backtrace"
-version = "0.3.45"
+version = "0.3.46"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ad235dabf00f36301792cfe82499880ba54c6486be094d1047b02bacb67c14e8"
+checksum = "b1e692897359247cc6bb902933361652380af0f1b7651ae5c5013407f30e109e"
 dependencies = [
  "backtrace-sys",
  "cfg-if",
@@ -184,9 +184,9 @@ dependencies = [
 
 [[package]]
 name = "backtrace-sys"
-version = "0.1.34"
+version = "0.1.35"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ca797db0057bae1a7aa2eef3283a874695455cecf08a43bfb8507ee0ebc1ed69"
+checksum = "7de8aba10a69c8e8d7622c5710229485ec32e9d55fdad160ea559c086fdcd118"
 dependencies = [
  "cc",
  "libc",
@@ -430,9 +430,9 @@ dependencies = [
 
 [[package]]
 name = "bumpalo"
-version = "3.2.0"
+version = "3.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1f359dc14ff8911330a51ef78022d376f25ed00248912803b58f00cb1c27f742"
+checksum = "12ae9db68ad7fac5fe51304d20f016c911539251075a214f8e663babefa35187"
 
 [[package]]
 name = "byte-slice-cast"
@@ -757,20 +757,11 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "69323bff1fb41c635347b8ead484a5ca6c3f11914d784170b158d8449ab07f8e"
 dependencies = [
  "cfg-if",
- "crossbeam-channel 0.4.2",
+ "crossbeam-channel",
  "crossbeam-deque",
  "crossbeam-epoch",
  "crossbeam-queue",
- "crossbeam-utils 0.7.2",
-]
-
-[[package]]
-name = "crossbeam-channel"
-version = "0.3.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c8ec7fcd21571dc78f96cc96243cab8d8f035247c3efd16c687be154c3fa9efa"
-dependencies = [
- "crossbeam-utils 0.6.6",
+ "crossbeam-utils",
 ]
 
 [[package]]
@@ -779,7 +770,7 @@ version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cced8691919c02aac3cb0a1bc2e9b73d89e832bf9a06fc579d4e71b68a2da061"
 dependencies = [
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "maybe-uninit",
 ]
 
@@ -790,7 +781,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9f02af974daeee82218205558e51ec8768b48cf524bd01d550abe5573a608285"
 dependencies = [
  "crossbeam-epoch",
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "maybe-uninit",
 ]
 
@@ -802,7 +793,7 @@ checksum = "058ed274caafc1f60c4997b5fc07bf7dc7cca454af7c6e81edffe5f33f70dace"
 dependencies = [
  "autocfg 1.0.0",
  "cfg-if",
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "lazy_static",
  "maybe-uninit",
  "memoffset",
@@ -816,17 +807,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c695eeca1e7173472a32221542ae469b3e9aac3a4fc81f7696bcad82029493db"
 dependencies = [
  "cfg-if",
- "crossbeam-utils 0.7.2",
-]
-
-[[package]]
-name = "crossbeam-utils"
-version = "0.6.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "04973fa96e96579258a5091af6003abde64af786b860f18622b82e026cca60e6"
-dependencies = [
- "cfg-if",
- "lazy_static",
+ "crossbeam-utils",
 ]
 
 [[package]]
@@ -1427,7 +1408,7 @@ checksum = "030a733c8287d6213886dd487564ff5c8f6aae10278b3588ed177f9d18f8d231"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
  "synstructure",
 ]
 
@@ -2136,7 +2117,7 @@ version = "0.13.0"
 source = "git+https://github.com/SigP/rust-libp2p?rev=44d7a9c9cd7be74109817bcabe74b991d5bd0fee#44d7a9c9cd7be74109817bcabe74b991d5bd0fee"
 dependencies = [
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
 ]
 
 [[package]]
@@ -2674,13 +2655,12 @@ dependencies = [
 [[package]]
 name = "milagro_bls"
 version = "1.0.1"
-source = "git+https://github.com/sigp/milagro_bls?branch=eth2.0-v0.10#9f946ca454ad1da4cd1344bd632b7306704bc904"
+source = "git+https://github.com/sigp/milagro_bls?branch=eth2.0-v0.10#38c6c33925b24c9319a1febfb621ff9bbf6d49f7"
 dependencies = [
  "amcl",
  "hex 0.4.2",
  "lazy_static",
  "rand 0.7.3",
- "yaml-rust",
  "zeroize 1.1.0",
 ]
 
@@ -3218,14 +3198,9 @@ dependencies = [
 
 [[package]]
 name = "proc-macro-hack"
-version = "0.5.12"
+version = "0.5.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f918f2b601f93baa836c1c2945faef682ba5b6d4828ecb45eeb7cc3c71b811b4"
-dependencies = [
- "proc-macro2 1.0.9",
- "quote 1.0.3",
- "syn 1.0.16",
-]
+checksum = "fcfdefadc3d57ca21cf17990a28ef4c0f7c61383a28cb7604cf4a18e6ede1420"
 
 [[package]]
 name = "proc-macro2"
@@ -3550,7 +3525,7 @@ checksum = "08a89b46efaf957e52b18062fb2f4660f8b8a4dde1807ca002690868ef2c85a9"
 dependencies = [
  "crossbeam-deque",
  "crossbeam-queue",
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "lazy_static",
  "num_cpus",
 ]
@@ -3583,9 +3558,9 @@ dependencies = [
 
 [[package]]
 name = "regex"
-version = "1.3.5"
+version = "1.3.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8900ebc1363efa7ea1c399ccc32daed870b4002651e0bed86e72d501ebbe0048"
+checksum = "7f6946991529684867e47d86474e3a6d0c0ab9b82d5821e314b1ede31fa3a4b3"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -3761,7 +3736,7 @@ dependencies = [
  "base64 0.11.0",
  "blake2b_simd",
  "constant_time_eq",
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
 ]
 
 [[package]]
@@ -3831,9 +3806,9 @@ dependencies = [
 
 [[package]]
 name = "schannel"
-version = "0.1.17"
+version = "0.1.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "507a9e6e8ffe0a4e0ebb9a10293e62fdf7657c06f1b8bb07a8fcf697d2abf295"
+checksum = "039c25b130bd8c1321ee2d7de7fde2659fa9c2744e4bb29711cfc852ea53cd19"
 dependencies = [
  "lazy_static",
  "winapi 0.3.8",
@@ -3918,22 +3893,22 @@ checksum = "a0eddf2e8f50ced781f288c19f18621fa72a3779e3cb58dbf23b07469b0abeb4"
 
 [[package]]
 name = "serde"
-version = "1.0.104"
+version = "1.0.105"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "414115f25f818d7dfccec8ee535d76949ae78584fc4f79a6f45a904bf8ab4449"
+checksum = "e707fbbf255b8fc8c3b99abb91e7257a622caeb20a9818cbadbeeede4e0932ff"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.104"
+version = "1.0.105"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "128f9e303a5a29922045a830221b8f78ec74a5f544944f3d5984f8ec3895ef64"
+checksum = "ac5d00fc561ba2724df6758a17de23df5914f20e41cb00f94d5b7ae42fffaff8"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
 ]
 
 [[package]]
@@ -3963,7 +3938,7 @@ checksum = "cd02c7587ec314570041b2754829f84d873ced14a96d1fd1823531e11db40573"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
 ]
 
 [[package]]
@@ -4081,11 +4056,11 @@ checksum = "1cc9c640a4adbfbcc11ffb95efe5aa7af7309e002adab54b185507dbf2377b99"
 
 [[package]]
 name = "slog-async"
-version = "2.4.0"
+version = "2.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "78ca925b180da88ccc595cbe4a3d378d79cb49fe5906c2cbc2488eaf700913ee"
+checksum = "51b3336ce47ce2f96673499fc07eb85e3472727b9a7a2959964b002c2ce8fbbb"
 dependencies = [
- "crossbeam-channel 0.3.9",
+ "crossbeam-channel",
  "slog",
  "take_mut",
  "thread_local",
@@ -4375,9 +4350,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "1.0.16"
+version = "1.0.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "123bd9499cfb380418d509322d7a6d52e5315f064fe4b3ad18a53d6b92c07859"
+checksum = "0df0eb663f387145cab623dea85b09c2c5b4b0aef44e945d928e682fce71bb03"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
@@ -4392,7 +4367,7 @@ checksum = "67656ea1dc1b41b1451851562ea232ec2e5a80242139f7e679ceccfb5d61f545"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
  "unicode-xid 0.2.0",
 ]
 
@@ -4613,7 +4588,7 @@ version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fb2d1b8f4548dbf5e1f7818512e9c406860678f29c300cdf0ebac72d1a3a1671"
 dependencies = [
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "futures",
 ]
 
@@ -4657,7 +4632,7 @@ version = "0.1.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "09bc590ec4ba8ba87652da2068d150dcada2cfa2e07faae270a5e0409aa51351"
 dependencies = [
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "futures",
  "lazy_static",
  "log 0.4.8",
@@ -4716,7 +4691,7 @@ checksum = "df720b6581784c118f0eb4310796b12b1d242a7eb95f716a8367855325c25f89"
 dependencies = [
  "crossbeam-deque",
  "crossbeam-queue",
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "futures",
  "lazy_static",
  "log 0.4.8",
@@ -4741,7 +4716,7 @@ version = "0.2.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "93044f2d313c95ff1cb7809ce9a7a05735b012288a888b62d4434fd58c94f296"
 dependencies = [
- "crossbeam-utils 0.7.2",
+ "crossbeam-utils",
  "futures",
  "slab 0.4.2",
  "tokio-executor",
@@ -4833,7 +4808,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "edcf0b9b2caa5f4804ef77aeee1b929629853d806117c48258f402b69737e65c"
 dependencies = [
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
 ]
 
 [[package]]
@@ -5189,7 +5164,7 @@ dependencies = [
  "log 0.4.8",
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
  "wasm-bindgen-shared",
 ]
 
@@ -5236,7 +5211,7 @@ checksum = "25bda4168030a6412ea8a047e27238cadf56f0e53516e1e83fec0a8b7c786f6d"
 dependencies = [
  "proc-macro2 1.0.9",
  "quote 1.0.3",
- "syn 1.0.16",
+ "syn 1.0.17",
  "wasm-bindgen-backend",
  "wasm-bindgen-shared",
 ]
diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 1065862f3..a084f1dae 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -80,6 +80,11 @@ pub enum AttestationType {
     Aggregated,
 }
 
+/// The accepted clock drift for nodes gossiping blocks and attestations (spec v0.11.0). See:
+///
+/// https://github.com/ethereum/eth2.0-specs/blob/v0.11.0/specs/phase0/p2p-interface.md#configuration
+pub const MAXIMUM_GOSSIP_CLOCK_DISPARITY: Duration = Duration::from_millis(500);
+
 #[derive(Debug, PartialEq)]
 pub enum AttestationProcessingOutcome {
     Processed,
diff --git a/beacon_node/beacon_chain/src/block_verification.rs b/beacon_node/beacon_chain/src/block_verification.rs
index cd4a8db52..0ef3b0550 100644
--- a/beacon_node/beacon_chain/src/block_verification.rs
+++ b/beacon_node/beacon_chain/src/block_verification.rs
@@ -42,10 +42,14 @@
 //! ```
 use crate::validator_pubkey_cache::ValidatorPubkeyCache;
 use crate::{
-    beacon_chain::{BLOCK_PROCESSING_CACHE_LOCK_TIMEOUT, VALIDATOR_PUBKEY_CACHE_LOCK_TIMEOUT},
+    beacon_chain::{
+        BLOCK_PROCESSING_CACHE_LOCK_TIMEOUT, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
+        VALIDATOR_PUBKEY_CACHE_LOCK_TIMEOUT,
+    },
     metrics, BeaconChain, BeaconChainError, BeaconChainTypes, BeaconSnapshot,
 };
 use parking_lot::RwLockReadGuard;
+use slot_clock::SlotClock;
 use state_processing::{
     block_signature_verifier::{
         BlockSignatureVerifier, Error as BlockSignatureVerifierError, G1Point,
@@ -259,19 +263,18 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
         chain: &BeaconChain<T>,
     ) -> Result<Self, BlockError> {
         // Do not gossip or process blocks from future slots.
-        //
-        // TODO: adjust this to allow for clock disparity tolerance.
-        let present_slot = chain.slot()?;
-        if block.slot() > present_slot {
+        let present_slot_with_tolerance = chain
+            .slot_clock
+            .now_with_future_tolerance(MAXIMUM_GOSSIP_CLOCK_DISPARITY)
+            .ok_or_else(|| BeaconChainError::UnableToReadSlot)?;
+        if block.slot() > present_slot_with_tolerance {
             return Err(BlockError::FutureSlot {
-                present_slot,
+                present_slot: present_slot_with_tolerance,
                 block_slot: block.slot(),
             });
         }
 
         // Do not gossip a block from a finalized slot.
-        //
-        // TODO: adjust this to allow for clock disparity tolerance.
         check_block_against_finalized_slot(&block.message, chain)?;
 
         // TODO: add check for the `(block.proposer_index, block.slot)` tuple once we have v0.11.0
diff --git a/beacon_node/beacon_chain/src/errors.rs b/beacon_node/beacon_chain/src/errors.rs
index 016aa33cc..2f18d41f8 100644
--- a/beacon_node/beacon_chain/src/errors.rs
+++ b/beacon_node/beacon_chain/src/errors.rs
@@ -72,8 +72,8 @@ easy_from_to!(SlotProcessingError, BeaconChainError);
 easy_from_to!(AttestationValidationError, BeaconChainError);
 easy_from_to!(SszTypesError, BeaconChainError);
 easy_from_to!(OpPoolError, BeaconChainError);
-easy_from_to!(BlockSignatureVerifierError, BeaconChainError);
 easy_from_to!(NaiveAggregationError, BeaconChainError);
+easy_from_to!(BlockSignatureVerifierError, BeaconChainError);
 
 #[derive(Debug, PartialEq)]
 pub enum BlockProductionError {
diff --git a/beacon_node/network/src/sync/range_sync/batch_processing.rs b/beacon_node/network/src/sync/range_sync/batch_processing.rs
index e518d0c4d..94ce6ae47 100644
--- a/beacon_node/network/src/sync/range_sync/batch_processing.rs
+++ b/beacon_node/network/src/sync/range_sync/batch_processing.rs
@@ -68,6 +68,8 @@ fn process_batch<T: BeaconChainTypes>(
                     log, "Parent block is unknown";
                     "parent_root" => format!("{}", parent),
                 );
+
+                return Err(format!("Block has an unknown parent: {}", parent));
             }
             Err(BlockError::BlockIsAlreadyKnown) => {
                 // this block is already known to us, move to the next
@@ -97,6 +99,11 @@ fn process_batch<T: BeaconChainTypes>(
                         "FUTURE_SLOT_TOLERANCE" => FUTURE_SLOT_TOLERANCE,
                     );
                 }
+
+                return Err(format!(
+                    "Block with slot {} is higher than the current slot {}",
+                    block_slot, present_slot
+                ));
             }
             Err(BlockError::WouldRevertFinalizedSlot { .. }) => {
                 debug!(
@@ -115,6 +122,8 @@ fn process_batch<T: BeaconChainTypes>(
                     "msg" => "unexpected condition in processing block.",
                     "outcome" => format!("{:?}", e)
                 );
+
+                return Err(format!("Internal error whilst processing block: {:?}", e));
             }
             other => {
                 warn!(
@@ -122,6 +131,8 @@ fn process_batch<T: BeaconChainTypes>(
                     "msg" => "peer sent invalid block",
                     "outcome" => format!("{:?}", other),
                 );
+
+                return Err(format!("Peer sent invalid block. Reason: {:?}", other));
             }
         }
     } else {