diff --git a/api/api_full.go b/api/api_full.go
index ca219dd27..12b3ecf63 100644
--- a/api/api_full.go
+++ b/api/api_full.go
@@ -949,7 +949,8 @@ const (
 )
 
 type Deadline struct {
-	PostSubmissions bitfield.BitField
+	PostSubmissions      bitfield.BitField
+	DisputableProofCount uint64
 }
 
 type Partition struct {
diff --git a/chain/actors/builtin/miner/miner.go b/chain/actors/builtin/miner/miner.go
index 38a5085db..49a468efb 100644
--- a/chain/actors/builtin/miner/miner.go
+++ b/chain/actors/builtin/miner/miner.go
@@ -107,6 +107,7 @@ type Deadline interface {
 	PartitionsPoSted() (bitfield.BitField, error)
 
 	PartitionsChanged(Deadline) (bool, error)
+	DisputableProofCount() (uint64, error)
 }
 
 type Partition interface {
diff --git a/chain/actors/builtin/miner/v0.go b/chain/actors/builtin/miner/v0.go
index 2795d6d70..ebe5cf085 100644
--- a/chain/actors/builtin/miner/v0.go
+++ b/chain/actors/builtin/miner/v0.go
@@ -391,6 +391,11 @@ func (d *deadline0) PartitionsPoSted() (bitfield.BitField, error) {
 	return d.Deadline.PostSubmissions, nil
 }
 
+func (d *deadline0) DisputableProofCount() (uint64, error) {
+	// field doesn't exist until v3
+	return 0, nil
+}
+
 func (p *partition0) AllSectors() (bitfield.BitField, error) {
 	return p.Partition.Sectors, nil
 }
diff --git a/chain/actors/builtin/miner/v2.go b/chain/actors/builtin/miner/v2.go
index 854aa6ab7..79f984213 100644
--- a/chain/actors/builtin/miner/v2.go
+++ b/chain/actors/builtin/miner/v2.go
@@ -390,6 +390,11 @@ func (d *deadline2) PartitionsPoSted() (bitfield.BitField, error) {
 	return d.Deadline.PostSubmissions, nil
 }
 
+func (d *deadline2) DisputableProofCount() (uint64, error) {
+	// field doesn't exist until v3
+	return 0, nil
+}
+
 func (p *partition2) AllSectors() (bitfield.BitField, error) {
 	return p.Partition.Sectors, nil
 }
diff --git a/chain/actors/builtin/miner/v3.go b/chain/actors/builtin/miner/v3.go
index 2d2ecacf4..3379e720e 100644
--- a/chain/actors/builtin/miner/v3.go
+++ b/chain/actors/builtin/miner/v3.go
@@ -386,6 +386,15 @@ func (d *deadline3) PartitionsPoSted() (bitfield.BitField, error) {
 	return d.Deadline.PartitionsPoSted, nil
 }
 
+func (d *deadline3) DisputableProofCount() (uint64, error) {
+	ops, err := d.OptimisticProofsSnapshotArray(d.store)
+	if err != nil {
+		return 0, err
+	}
+
+	return ops.Length(), nil
+}
+
 func (p *partition3) AllSectors() (bitfield.BitField, error) {
 	return p.Partition.Sectors, nil
 }
diff --git a/cli/chain.go b/cli/chain.go
index 2dc0801b4..539ad1a79 100644
--- a/cli/chain.go
+++ b/cli/chain.go
@@ -58,6 +58,7 @@ var chainCmd = &cli.Command{
 		chainInspectUsage,
 		chainDecodeCmd,
 		chainEncodeCmd,
+		chainDisputeSetCmd,
 	},
 }
 
diff --git a/cli/disputer.go b/cli/disputer.go
new file mode 100644
index 000000000..40a3092ca
--- /dev/null
+++ b/cli/disputer.go
@@ -0,0 +1,429 @@
+package cli
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"time"
+
+	"github.com/filecoin-project/go-state-types/abi"
+
+	"github.com/filecoin-project/go-address"
+
+	"github.com/filecoin-project/lotus/chain/actors"
+
+	miner3 "github.com/filecoin-project/specs-actors/v3/actors/builtin/miner"
+
+	"github.com/filecoin-project/go-state-types/big"
+	lapi "github.com/filecoin-project/lotus/api"
+	"github.com/filecoin-project/lotus/chain/types"
+	builtin3 "github.com/filecoin-project/specs-actors/v3/actors/builtin"
+	"golang.org/x/xerrors"
+
+	"github.com/filecoin-project/lotus/chain/store"
+	"github.com/urfave/cli/v2"
+)
+
+const Confidence = 10
+
+type minerDeadline struct {
+	miner address.Address
+	index uint64
+}
+
+var chainDisputeSetCmd = &cli.Command{
+	Name:  "disputer",
+	Usage: "interact with the window post disputer",
+	Flags: []cli.Flag{
+		&cli.StringFlag{
+			Name:  "max-fee",
+			Usage: "Spend up to X FIL per DisputeWindowedPoSt message",
+		},
+		&cli.StringFlag{
+			Name:  "from",
+			Usage: "optionally specify the account to send messages from",
+		},
+	},
+	Subcommands: []*cli.Command{
+		disputerStartCmd,
+		disputerMsgCmd,
+	},
+}
+
+var disputerMsgCmd = &cli.Command{
+	Name:      "dispute",
+	Usage:     "Send a specific DisputeWindowedPoSt message",
+	ArgsUsage: "[minerAddress index postIndex]",
+	Flags:     []cli.Flag{},
+	Action: func(cctx *cli.Context) error {
+		if cctx.NArg() != 3 {
+			fmt.Println("Usage: dispute [minerAddress index postIndex]")
+			return nil
+		}
+
+		ctx := ReqContext(cctx)
+
+		api, closer, err := GetFullNodeAPI(cctx)
+		if err != nil {
+			return err
+		}
+		defer closer()
+
+		toa, err := address.NewFromString(cctx.Args().First())
+		if err != nil {
+			return fmt.Errorf("given 'miner' address %q was invalid: %w", cctx.Args().First(), err)
+		}
+
+		deadline, err := strconv.ParseUint(cctx.Args().Get(1), 10, 64)
+		if err != nil {
+			return err
+		}
+
+		postIndex, err := strconv.ParseUint(cctx.Args().Get(2), 10, 64)
+		if err != nil {
+			return err
+		}
+
+		fromAddr, err := getSender(ctx, api, cctx.String("from"))
+		if err != nil {
+			return err
+		}
+
+		dpp, aerr := actors.SerializeParams(&miner3.DisputeWindowedPoStParams{
+			Deadline:  deadline,
+			PoStIndex: postIndex,
+		})
+
+		if aerr != nil {
+			return xerrors.Errorf("failed to serailize params: %w", aerr)
+		}
+
+		dmsg := &types.Message{
+			To:     toa,
+			From:   fromAddr,
+			Value:  big.Zero(),
+			Method: builtin3.MethodsMiner.DisputeWindowedPoSt,
+			Params: dpp,
+		}
+
+		rslt, err := api.StateCall(ctx, dmsg, types.EmptyTSK)
+		if err != nil {
+			return xerrors.Errorf("failed to simulate dispute: %w", err)
+		}
+
+		if rslt.MsgRct.ExitCode == 0 {
+			mss, err := getMaxFee(cctx.String("max-fee"))
+			if err != nil {
+				return err
+			}
+
+			sm, err := api.MpoolPushMessage(ctx, dmsg, mss)
+			if err != nil {
+				return err
+			}
+
+			fmt.Println("dispute message ", sm.Cid())
+		} else {
+			fmt.Println("dispute is unsuccessful")
+		}
+
+		return nil
+	},
+}
+
+var disputerStartCmd = &cli.Command{
+	Name:      "start",
+	Usage:     "Start the window post disputer",
+	ArgsUsage: "[minerAddress]",
+	Flags: []cli.Flag{
+		&cli.Uint64Flag{
+			Name:  "start-epoch",
+			Usage: "only start disputing PoSts after this epoch ",
+		},
+	},
+	Action: func(cctx *cli.Context) error {
+		api, closer, err := GetFullNodeAPI(cctx)
+		if err != nil {
+			return err
+		}
+		defer closer()
+
+		ctx := ReqContext(cctx)
+
+		fromAddr, err := getSender(ctx, api, cctx.String("from"))
+		if err != nil {
+			return err
+		}
+
+		mss, err := getMaxFee(cctx.String("max-fee"))
+		if err != nil {
+			return err
+		}
+
+		startEpoch := abi.ChainEpoch(0)
+		if cctx.IsSet("height") {
+			startEpoch = abi.ChainEpoch(cctx.Uint64("height"))
+		}
+
+		fmt.Println("checking sync status")
+
+		if err := SyncWait(ctx, api, false); err != nil {
+			return xerrors.Errorf("sync wait: %w", err)
+		}
+
+		fmt.Println("setting up window post disputer")
+
+		// subscribe to head changes and validate the current value
+
+		headChanges, err := api.ChainNotify(ctx)
+		if err != nil {
+			return err
+		}
+
+		head, ok := <-headChanges
+		if !ok {
+			return xerrors.Errorf("Notify stream was invalid")
+		}
+
+		if len(head) != 1 {
+			return xerrors.Errorf("Notify first entry should have been one item")
+		}
+
+		if head[0].Type != store.HCCurrent {
+			return xerrors.Errorf("expected current head on Notify stream (got %s)", head[0].Type)
+		}
+
+		lastEpoch := head[0].Val.Height()
+		lastStatusCheckEpoch := lastEpoch
+
+		// build initial deadlineMap
+
+		minerList, err := api.StateListMiners(ctx, types.EmptyTSK)
+		if err != nil {
+			return err
+		}
+
+		knownMiners := make(map[address.Address]struct{})
+		deadlineMap := make(map[abi.ChainEpoch][]minerDeadline)
+		for _, miner := range minerList {
+			dClose, dl, err := makeMinerDeadline(ctx, api, miner)
+			if err != nil {
+				return xerrors.Errorf("making deadline: %w", err)
+			}
+
+			deadlineMap[dClose+Confidence] = append(deadlineMap[dClose+Confidence], *dl)
+
+			knownMiners[miner] = struct{}{}
+		}
+
+		// when this fires, check for newly created miners, and purge any "missed" epochs from deadlineMap
+		statusCheckTicker := time.NewTicker(time.Hour)
+		defer statusCheckTicker.Stop()
+
+		fmt.Println("starting up window post disputer")
+
+		applyTsk := func(tsk types.TipSetKey) error {
+			log.Infof("last checked height: %d", lastEpoch)
+			dls, ok := deadlineMap[lastEpoch]
+			delete(deadlineMap, lastEpoch)
+			if !ok || startEpoch >= lastEpoch {
+				// no deadlines closed at this epoch - Confidence, or we haven't reached the start cutoff yet
+				return nil
+			}
+
+			dpmsgs := make([]*types.Message, 0)
+
+			// TODO: Parallelizeable
+			for _, dl := range dls {
+				fullDeadlines, err := api.StateMinerDeadlines(ctx, dl.miner, tsk)
+				if err != nil {
+					return xerrors.Errorf("failed to load deadlines: %w", err)
+				}
+
+				if int(dl.index) >= len(fullDeadlines) {
+					return xerrors.Errorf("deadline index %d not found in deadlines", dl.index)
+				}
+
+				ms, err := makeDisputeWindowedPosts(ctx, api, dl, fullDeadlines[dl.index].DisputableProofCount, fromAddr)
+				if err != nil {
+					return xerrors.Errorf("failed to check for disputes: %w", err)
+				}
+
+				dpmsgs = append(dpmsgs, ms...)
+
+				dClose, dl, err := makeMinerDeadline(ctx, api, dl.miner)
+				if err != nil {
+					return xerrors.Errorf("making deadline: %w", err)
+				}
+
+				deadlineMap[dClose+Confidence] = append(deadlineMap[dClose+Confidence], *dl)
+			}
+
+			// TODO: Parallelizeable / can be integrated into the previous deadline-iterating for loop
+			for _, dpmsg := range dpmsgs {
+				log.Infof("disputing a PoSt from miner %s", dpmsg.To)
+				m, err := api.MpoolPushMessage(ctx, dpmsg, mss)
+				if err != nil {
+					log.Infof("failed to dispute post message: %s", err.Error())
+				} else {
+					log.Infof("disputed a PoSt in message: %s", m.Cid())
+				}
+			}
+
+			return nil
+		}
+
+		disputeLoop := func() error {
+			select {
+			case notif, ok := <-headChanges:
+				if !ok {
+					return xerrors.Errorf("head change channel errored")
+				}
+
+				for _, val := range notif {
+					switch val.Type {
+					case store.HCApply:
+						for ; lastEpoch <= val.Val.Height(); lastEpoch++ {
+							err := applyTsk(val.Val.Key())
+							if err != nil {
+								return err
+							}
+						}
+					case store.HCRevert:
+						// do nothing
+					default:
+						return xerrors.Errorf("unexpected head change type %s", val.Type)
+					}
+				}
+			case <-statusCheckTicker.C:
+				log.Infof("Running status check: ")
+
+				minerList, err = api.StateListMiners(ctx, types.EmptyTSK)
+				if err != nil {
+					return xerrors.Errorf("getting miner list: %w", err)
+				}
+
+				for _, m := range minerList {
+					_, ok := knownMiners[m]
+					if !ok {
+						dClose, dl, err := makeMinerDeadline(ctx, api, m)
+						if err != nil {
+							return xerrors.Errorf("making deadline: %w", err)
+						}
+
+						deadlineMap[dClose+Confidence] = append(deadlineMap[dClose+Confidence], *dl)
+
+						knownMiners[m] = struct{}{}
+					}
+				}
+
+				for ; lastStatusCheckEpoch < lastEpoch; lastStatusCheckEpoch++ {
+					// if an epoch got "skipped" from the deadlineMap somehow, just fry it now instead of letting it sit around forever
+					_, ok := deadlineMap[lastStatusCheckEpoch]
+					if ok {
+						log.Infof("epoch %d was skipped during execution, deleting it from deadlineMap")
+						delete(deadlineMap, lastStatusCheckEpoch)
+					}
+				}
+
+				log.Infof("Status check complete")
+			case <-ctx.Done():
+				return xerrors.Errorf("context cancelled")
+			}
+
+			return nil
+		}
+
+		for {
+			err := disputeLoop()
+			if err != nil {
+				fmt.Println("disputer shutting down: ", err)
+				break
+			}
+		}
+
+		return nil
+	},
+}
+
+// for a given miner, index, and maxPostIndex, tries to dispute posts from 0...postsSnapshotted-1
+// returns a list of DisputeWindowedPoSt msgs that are expected to succeed if sent
+func makeDisputeWindowedPosts(ctx context.Context, api lapi.FullNode, dl minerDeadline, postsSnapshotted uint64, sender address.Address) ([]*types.Message, error) {
+	disputes := make([]*types.Message, 0)
+
+	for i := uint64(0); i < postsSnapshotted; i++ {
+
+		dpp, aerr := actors.SerializeParams(&miner3.DisputeWindowedPoStParams{
+			Deadline:  dl.index,
+			PoStIndex: i,
+		})
+
+		if aerr != nil {
+			return nil, xerrors.Errorf("failed to serailize params: %w", aerr)
+		}
+
+		dispute := &types.Message{
+			To:     dl.miner,
+			From:   sender,
+			Value:  big.Zero(),
+			Method: builtin3.MethodsMiner.DisputeWindowedPoSt,
+			Params: dpp,
+		}
+
+		rslt, err := api.StateCall(ctx, dispute, types.EmptyTSK)
+		if err == nil && rslt.MsgRct.ExitCode == 0 {
+			disputes = append(disputes, dispute)
+		}
+
+	}
+
+	return disputes, nil
+}
+
+func makeMinerDeadline(ctx context.Context, api lapi.FullNode, mAddr address.Address) (abi.ChainEpoch, *minerDeadline, error) {
+	dl, err := api.StateMinerProvingDeadline(ctx, mAddr, types.EmptyTSK)
+	if err != nil {
+		return -1, nil, xerrors.Errorf("getting proving index list: %w", err)
+	}
+
+	return dl.Close, &minerDeadline{
+		miner: mAddr,
+		index: dl.Index,
+	}, nil
+}
+
+func getSender(ctx context.Context, api lapi.FullNode, fromStr string) (address.Address, error) {
+	if fromStr == "" {
+		return api.WalletDefaultAddress(ctx)
+	}
+
+	addr, err := address.NewFromString(fromStr)
+	if err != nil {
+		return address.Undef, err
+	}
+
+	has, err := api.WalletHas(ctx, addr)
+	if err != nil {
+		return address.Undef, err
+	}
+
+	if !has {
+		return address.Undef, xerrors.Errorf("wallet doesn't contain: %s ", addr)
+	}
+
+	return addr, nil
+}
+
+func getMaxFee(maxStr string) (*lapi.MessageSendSpec, error) {
+	if maxStr != "" {
+		maxFee, err := types.ParseFIL(maxStr)
+		if err != nil {
+			return nil, xerrors.Errorf("parsing max-fee: %w", err)
+		}
+		return &lapi.MessageSendSpec{
+			MaxFee: types.BigInt(maxFee),
+		}, nil
+	}
+
+	return nil, nil
+}
diff --git a/node/impl/full/state.go b/node/impl/full/state.go
index cecf8ef55..91a1b74db 100644
--- a/node/impl/full/state.go
+++ b/node/impl/full/state.go
@@ -170,8 +170,14 @@ func (a *StateAPI) StateMinerDeadlines(ctx context.Context, m address.Address, t
 			return err
 		}
 
+		l, err := dl.DisputableProofCount()
+		if err != nil {
+			return err
+		}
+
 		out[i] = api.Deadline{
-			PostSubmissions: ps,
+			PostSubmissions:      ps,
+			DisputableProofCount: l,
 		}
 		return nil
 	}); err != nil {