diff --git a/blocks/block.go b/blocks/block.go
index 77d636d89..15518d22f 100644
--- a/blocks/block.go
+++ b/blocks/block.go
@@ -4,7 +4,9 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"errors"
+	"fmt"
 	"io"
+	"strings"
 	"time"
 
 	. "github.com/tendermint/tendermint/binary"
@@ -58,17 +60,16 @@ func (b *Block) ValidateBasic(lastBlockHeight uint32, lastBlockHash []byte) erro
 }
 
 func (b *Block) Hash() []byte {
-	if b.hash != nil {
-		return b.hash
-	} else {
+	if b.hash == nil {
 		hashes := [][]byte{
 			b.Header.Hash(),
 			b.Validation.Hash(),
 			b.Data.Hash(),
 		}
 		// Merkle hash from sub-hashes.
-		return merkle.HashFromHashes(hashes)
+		b.hash = merkle.HashFromHashes(hashes)
 	}
+	return b.hash
 }
 
 // Convenience.
@@ -88,27 +89,40 @@ func (b *Block) HashesTo(hash []byte) bool {
 func (b *Block) MakeNextBlock() *Block {
 	return &Block{
 		Header: Header{
-			Network: b.Header.Network,
-			Height:  b.Header.Height + 1,
-			//Fees:                uint64(0),
+			Network:       b.Header.Network,
+			Height:        b.Header.Height + 1,
 			Time:          time.Now(),
 			LastBlockHash: b.Hash(),
-			//ValidationStateHash: nil,
-			//AccountStateHash:    nil,
+			StateHash:     nil,
 		},
 	}
 }
 
+func (b *Block) String() string {
+	return b.StringWithIndent("")
+}
+
+func (b *Block) StringWithIndent(indent string) string {
+	return fmt.Sprintf(`Block{
+%s  %v
+%s  %v
+%s  %v
+%s}#%X`,
+		indent, b.Header.StringWithIndent(indent+"  "),
+		indent, b.Validation.StringWithIndent(indent+"  "),
+		indent, b.Data.StringWithIndent(indent+"  "),
+		indent, b.hash)
+}
+
 //-----------------------------------------------------------------------------
 
 type Header struct {
-	Network             string
-	Height              uint32
-	Fees                uint64
-	Time                time.Time
-	LastBlockHash       []byte
-	ValidationStateHash []byte
-	AccountStateHash    []byte
+	Network       string
+	Height        uint32
+	Time          time.Time
+	Fees          uint64
+	LastBlockHash []byte
+	StateHash     []byte
 
 	// Volatile
 	hash []byte
@@ -119,39 +133,53 @@ func ReadHeader(r io.Reader, n *int64, err *error) (h Header) {
 		return Header{}
 	}
 	return Header{
-		Network:             ReadString(r, n, err),
-		Height:              ReadUInt32(r, n, err),
-		Fees:                ReadUInt64(r, n, err),
-		Time:                ReadTime(r, n, err),
-		LastBlockHash:       ReadByteSlice(r, n, err),
-		ValidationStateHash: ReadByteSlice(r, n, err),
-		AccountStateHash:    ReadByteSlice(r, n, err),
+		Network:       ReadString(r, n, err),
+		Height:        ReadUInt32(r, n, err),
+		Time:          ReadTime(r, n, err),
+		Fees:          ReadUInt64(r, n, err),
+		LastBlockHash: ReadByteSlice(r, n, err),
+		StateHash:     ReadByteSlice(r, n, err),
 	}
 }
 
 func (h *Header) WriteTo(w io.Writer) (n int64, err error) {
 	WriteString(w, h.Network, &n, &err)
 	WriteUInt32(w, h.Height, &n, &err)
-	WriteUInt64(w, h.Fees, &n, &err)
 	WriteTime(w, h.Time, &n, &err)
+	WriteUInt64(w, h.Fees, &n, &err)
 	WriteByteSlice(w, h.LastBlockHash, &n, &err)
-	WriteByteSlice(w, h.ValidationStateHash, &n, &err)
-	WriteByteSlice(w, h.AccountStateHash, &n, &err)
+	WriteByteSlice(w, h.StateHash, &n, &err)
 	return
 }
 
 func (h *Header) Hash() []byte {
-	if h.hash != nil {
-		return h.hash
-	} else {
+	if h.hash == nil {
 		hasher := sha256.New()
 		_, err := h.WriteTo(hasher)
 		if err != nil {
 			panic(err)
 		}
 		h.hash = hasher.Sum(nil)
-		return h.hash
 	}
+	return h.hash
+}
+
+func (h *Header) StringWithIndent(indent string) string {
+	return fmt.Sprintf(`Header{
+%s  Network:       %v
+%s  Height:        %v
+%s  Time:          %v
+%s  Fees:          %v
+%s  LastBlockHash: %X
+%s  StateHash:     %X
+%s}#%X`,
+		indent, h.Network,
+		indent, h.Height,
+		indent, h.Time,
+		indent, h.Fees,
+		indent, h.LastBlockHash,
+		indent, h.StateHash,
+		indent, h.hash)
 }
 
 //-----------------------------------------------------------------------------
@@ -183,17 +211,26 @@ func (v *Validation) WriteTo(w io.Writer) (n int64, err error) {
 }
 
 func (v *Validation) Hash() []byte {
-	if v.hash != nil {
-		return v.hash
-	} else {
-		hasher := sha256.New()
-		_, err := v.WriteTo(hasher)
-		if err != nil {
-			panic(err)
+	if v.hash == nil {
+		bs := make([]Binary, len(v.Signatures))
+		for i, sig := range v.Signatures {
+			bs[i] = Binary(sig)
 		}
-		v.hash = hasher.Sum(nil)
-		return v.hash
+		v.hash = merkle.HashFromBinaries(bs)
+	}
+	return v.hash
+}
+
+func (v *Validation) StringWithIndent(indent string) string {
+	sigStrings := make([]string, len(v.Signatures))
+	for i, sig := range v.Signatures {
+		sigStrings[i] = sig.String()
 	}
+	return fmt.Sprintf(`Validation{
+%s  %v
+%s}#%X`,
+		indent, strings.Join(sigStrings, "\n"+indent+"  "),
+		indent, v.hash)
 }
 
 //-----------------------------------------------------------------------------
@@ -223,14 +260,24 @@ func (data *Data) WriteTo(w io.Writer) (n int64, err error) {
 }
 
 func (data *Data) Hash() []byte {
-	if data.hash != nil {
-		return data.hash
-	} else {
-		bs := make([]Binary, 0, len(data.Txs))
+	if data.hash == nil {
+		bs := make([]Binary, len(data.Txs))
 		for i, tx := range data.Txs {
 			bs[i] = Binary(tx)
 		}
 		data.hash = merkle.HashFromBinaries(bs)
-		return data.hash
 	}
+	return data.hash
+}
+
+func (data *Data) StringWithIndent(indent string) string {
+	txStrings := make([]string, len(data.Txs))
+	for i, tx := range data.Txs {
+		txStrings[i] = tx.String()
+	}
+	return fmt.Sprintf(`Data{
+%s  %v
+%s}#%X`,
+		indent, strings.Join(txStrings, "\n"+indent+"  "),
+		indent, data.hash)
 }
diff --git a/blocks/block_test.go b/blocks/block_test.go
index 5c93897c8..474fa9f8a 100644
--- a/blocks/block_test.go
+++ b/blocks/block_test.go
@@ -15,16 +15,13 @@ func randBaseTx() BaseTx {
 	return BaseTx{0, RandUInt64Exp(), randSig()}
 }
 
-func TestBlock(t *testing.T) {
-
+func randBlock() *Block {
 	// Account Txs
-
 	sendTx := &SendTx{
 		BaseTx: randBaseTx(),
 		To:     RandUInt64Exp(),
 		Amount: RandUInt64Exp(),
 	}
-
 	nameTx := &NameTx{
 		BaseTx: randBaseTx(),
 		Name:   string(RandBytes(12)),
@@ -32,16 +29,13 @@ func TestBlock(t *testing.T) {
 	}
 
 	// Validation Txs
-
 	bondTx := &BondTx{
 		BaseTx: randBaseTx(),
 		//UnbondTo: RandUInt64Exp(),
 	}
-
 	unbondTx := &UnbondTx{
 		BaseTx: randBaseTx(),
 	}
-
 	dupeoutTx := &DupeoutTx{
 		VoteA: Vote{
 			Height:    RandUInt32Exp(),
@@ -60,16 +54,14 @@ func TestBlock(t *testing.T) {
 	}
 
 	// Block
-
 	block := &Block{
 		Header: Header{
-			Network:             "Tendermint",
-			Height:              RandUInt32Exp(),
-			Fees:                RandUInt64Exp(),
-			Time:                RandTime(),
-			LastBlockHash:       RandBytes(32),
-			ValidationStateHash: RandBytes(32),
-			AccountStateHash:    RandBytes(32),
+			Network:       "Tendermint",
+			Height:        RandUInt32Exp(),
+			Fees:          RandUInt64Exp(),
+			Time:          RandTime(),
+			LastBlockHash: RandBytes(32),
+			StateHash:     RandBytes(32),
 		},
 		Validation: Validation{
 			Signatures: []Signature{randSig(), randSig()},
@@ -78,22 +70,52 @@ func TestBlock(t *testing.T) {
 			Txs: []Tx{sendTx, nameTx, bondTx, unbondTx, dupeoutTx},
 		},
 	}
+	return block
+}
 
-	// Write the block, read it in again, write it again.
-	// Then, compare.
-	// TODO We should compute the hash instead, so Block -> Bytes -> Block and compare hashes.
+func TestBlock(t *testing.T) {
 
-	blockBytes := BinaryBytes(block)
+	block := randBlock()
+	// Mutate the block and ensure that the hash changed.
+	lastHash := block.Hash()
+	expectChange := func(mutateFn func(b *Block), message string) {
+		// mutate block
+		mutateFn(block)
+		// nuke hashes
+		block.hash = nil
+		block.Header.hash = nil
+		block.Validation.hash = nil
+		block.Data.hash = nil
+		// compare
+		if bytes.Equal(lastHash, block.Hash()) {
+			t.Error(message)
+		} else {
+			lastHash = block.Hash()
+		}
+	}
+	expectChange(func(b *Block) { b.Header.Network = "blah" }, "Expected hash to depend on Network")
+	expectChange(func(b *Block) { b.Header.Height += 1 }, "Expected hash to depend on Height")
+	expectChange(func(b *Block) { b.Header.Fees += 1 }, "Expected hash to depend on Fees")
+	expectChange(func(b *Block) { b.Header.Time = RandTime() }, "Expected hash to depend on Time")
+	expectChange(func(b *Block) { b.Header.LastBlockHash = RandBytes(32) }, "Expected hash to depend on LastBlockHash")
+	expectChange(func(b *Block) { b.Header.StateHash = RandBytes(32) }, "Expected hash to depend on StateHash")
+	expectChange(func(b *Block) { b.Validation.Signatures[0].SignerId += 1 }, "Expected hash to depend on Validation Signature")
+	expectChange(func(b *Block) { b.Validation.Signatures[0].Bytes = RandBytes(32) }, "Expected hash to depend on Validation Signature")
+	expectChange(func(b *Block) { b.Data.Txs[0].(*SendTx).SignerId += 1 }, "Expected hash to depend on tx Signature")
+	expectChange(func(b *Block) { b.Data.Txs[0].(*SendTx).Amount += 1 }, "Expected hash to depend on send tx Amount")
+
+	// Write the block, read it in again, check hash.
+	block1 := randBlock()
+	block1Bytes := BinaryBytes(block1)
 	var n int64
 	var err error
-	block2 := ReadBlock(bytes.NewReader(blockBytes), &n, &err)
+	block2 := ReadBlock(bytes.NewReader(block1Bytes), &n, &err)
 	if err != nil {
 		t.Errorf("Reading block failed: %v", err)
 	}
-
-	blockBytes2 := BinaryBytes(block2)
-
-	if !bytes.Equal(blockBytes, blockBytes2) {
-		t.Fatal("Write->Read of block failed.")
+	if !bytes.Equal(block1.Hash(), block2.Hash()) {
+		t.Errorf("Expected write/read to preserve original hash")
+		t.Logf("\nBlock1:\n%v", block1)
+		t.Logf("\nBlock2:\n%v", block2)
 	}
 }
diff --git a/blocks/codec_test.go b/blocks/codec_test.go
index 1339f1b19..32c10769d 100644
--- a/blocks/codec_test.go
+++ b/blocks/codec_test.go
@@ -15,13 +15,12 @@ func BenchmarkTestCustom(b *testing.B) {
 	b.StopTimer()
 
 	h := &Header{
-		Network:             "Header",
-		Height:              123,
-		Fees:                123,
-		Time:                time.Unix(123, 0),
-		LastBlockHash:       []byte("prevhash"),
-		ValidationStateHash: []byte("validationhash"),
-		AccountStateHash:    []byte("accounthash"),
+		Network:       "Header",
+		Height:        123,
+		Fees:          123,
+		Time:          time.Unix(123, 0),
+		LastBlockHash: []byte("prevhash"),
+		StateHash:     []byte("statehash"),
 	}
 
 	buf := bytes.NewBuffer(nil)
@@ -40,26 +39,24 @@ func BenchmarkTestCustom(b *testing.B) {
 }
 
 type HHeader struct {
-	Network             string `json:"N"`
-	Height              uint64 `json:"H"`
-	Fees                uint64 `json:"F"`
-	Time                uint64 `json:"T"`
-	LastBlockHash       []byte `json:"PH"`
-	ValidationStateHash []byte `json:"VH"`
-	AccountStateHash    []byte `json:"DH"`
+	Network       string `json:"N"`
+	Height        uint64 `json:"H"`
+	Fees          uint64 `json:"F"`
+	Time          uint64 `json:"T"`
+	LastBlockHash []byte `json:"PH"`
+	StateHash     []byte `json:"SH"`
 }
 
 func BenchmarkTestJSON(b *testing.B) {
 	b.StopTimer()
 
 	h := &HHeader{
-		Network:             "Header",
-		Height:              123,
-		Fees:                123,
-		Time:                123,
-		LastBlockHash:       []byte("prevhash"),
-		ValidationStateHash: []byte("validationhash"),
-		AccountStateHash:    []byte("accounthash"),
+		Network:       "Header",
+		Height:        123,
+		Fees:          123,
+		Time:          123,
+		LastBlockHash: []byte("prevhash"),
+		StateHash:     []byte("statehash"),
 	}
 	h2 := &HHeader{}
 
@@ -82,13 +79,12 @@ func BenchmarkTestGob(b *testing.B) {
 	b.StopTimer()
 
 	h := &Header{
-		Network:             "Header",
-		Height:              123,
-		Fees:                123,
-		Time:                time.Unix(123, 0),
-		LastBlockHash:       []byte("prevhash"),
-		ValidationStateHash: []byte("validationhash"),
-		AccountStateHash:    []byte("datahash"),
+		Network:       "Header",
+		Height:        123,
+		Fees:          123,
+		Time:          time.Unix(123, 0),
+		LastBlockHash: []byte("prevhash"),
+		StateHash:     []byte("statehash"),
 	}
 	h2 := &Header{}
 
@@ -111,13 +107,12 @@ func BenchmarkTestMsgPack(b *testing.B) {
 	b.StopTimer()
 
 	h := &Header{
-		Network:             "Header",
-		Height:              123,
-		Fees:                123,
-		Time:                time.Unix(123, 0),
-		LastBlockHash:       []byte("prevhash"),
-		ValidationStateHash: []byte("validationhash"),
-		AccountStateHash:    []byte("datahash"),
+		Network:       "Header",
+		Height:        123,
+		Fees:          123,
+		Time:          time.Unix(123, 0),
+		LastBlockHash: []byte("prevhash"),
+		StateHash:     []byte("statehash"),
 	}
 	h2 := &Header{}
 
@@ -140,13 +135,12 @@ func BenchmarkTestMsgPack2(b *testing.B) {
 	b.StopTimer()
 
 	h := &Header{
-		Network:             "Header",
-		Height:              123,
-		Fees:                123,
-		Time:                time.Unix(123, 0),
-		LastBlockHash:       []byte("prevhash"),
-		ValidationStateHash: []byte("validationhash"),
-		AccountStateHash:    []byte("accounthash"),
+		Network:       "Header",
+		Height:        123,
+		Fees:          123,
+		Time:          time.Unix(123, 0),
+		LastBlockHash: []byte("prevhash"),
+		StateHash:     []byte("statehash"),
 	}
 	h2 := &Header{}
 	var mh codec.MsgpackHandle
diff --git a/blocks/signature.go b/blocks/signature.go
index 33f6f9a47..7c73328b9 100644
--- a/blocks/signature.go
+++ b/blocks/signature.go
@@ -1,8 +1,10 @@
 package blocks
 
 import (
-	. "github.com/tendermint/tendermint/binary"
+	"fmt"
 	"io"
+
+	. "github.com/tendermint/tendermint/binary"
 )
 
 type Signable interface {
@@ -35,6 +37,12 @@ func (sig Signature) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+func (sig Signature) String() string {
+	return fmt.Sprintf("Signature{%v:%X}", sig.SignerId, sig.Bytes)
+}
+
+//-------------------------------------
+
 func ReadSignatures(r io.Reader, n *int64, err *error) (sigs []Signature) {
 	length := ReadUInt32(r, n, err)
 	for i := uint32(0); i < length; i++ {
diff --git a/blocks/tx.go b/blocks/tx.go
index a262d6854..421a599e7 100644
--- a/blocks/tx.go
+++ b/blocks/tx.go
@@ -1,9 +1,11 @@
 package blocks
 
 import (
+	"fmt"
+	"io"
+
 	. "github.com/tendermint/tendermint/binary"
 	. "github.com/tendermint/tendermint/common"
-	"io"
 )
 
 /*
@@ -21,6 +23,7 @@ type Tx interface {
 	Signable
 	GetSequence() uint
 	GetFee() uint64
+	String() string
 }
 
 const (
@@ -108,6 +111,10 @@ func (tx *BaseTx) SetSignature(sig Signature) {
 	tx.Signature = sig
 }
 
+func (tx *BaseTx) String() string {
+	return fmt.Sprintf("{S:%v F:%v Sig:%X}", tx.Sequence, tx.Fee, tx.Signature)
+}
+
 //-----------------------------------------------------------------------------
 
 type SendTx struct {
@@ -124,6 +131,10 @@ func (tx *SendTx) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+func (tx *SendTx) String() string {
+	return fmt.Sprintf("SendTx{%v To:%v Amount:%v}", tx.BaseTx, tx.To, tx.Amount)
+}
+
 //-----------------------------------------------------------------------------
 
 type NameTx struct {
@@ -140,6 +151,10 @@ func (tx *NameTx) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+func (tx *NameTx) String() string {
+	return fmt.Sprintf("NameTx{%v Name:%v PubKey:%X}", tx.BaseTx, tx.Name, tx.PubKey)
+}
+
 //-----------------------------------------------------------------------------
 
 type BondTx struct {
@@ -154,6 +169,10 @@ func (tx *BondTx) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+func (tx *BondTx) String() string {
+	return fmt.Sprintf("BondTx{%v}", tx.BaseTx)
+}
+
 //-----------------------------------------------------------------------------
 
 type UnbondTx struct {
@@ -166,6 +185,10 @@ func (tx *UnbondTx) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+func (tx *UnbondTx) String() string {
+	return fmt.Sprintf("UnbondTx{%v}", tx.BaseTx)
+}
+
 //-----------------------------------------------------------------------------
 
 type DupeoutTx struct {
@@ -181,3 +204,7 @@ func (tx *DupeoutTx) WriteTo(w io.Writer) (n int64, err error) {
 	WriteBinary(w, &tx.VoteB, &n, &err)
 	return
 }
+
+func (tx *DupeoutTx) String() string {
+	return fmt.Sprintf("DupeoutTx{%v VoteA:%v VoteB:%v}", tx.BaseTx, tx.VoteA, tx.VoteB)
+}
diff --git a/mempool/mempool.go b/mempool/mempool.go
index b721cde21..c2b2b2b68 100644
--- a/mempool/mempool.go
+++ b/mempool/mempool.go
@@ -53,8 +53,10 @@ func (mem *Mempool) MakeProposalBlock() (*Block, *State) {
 	return nextBlock, mem.state
 }
 
-// Txs that are present in block are discarded from mempool.
-// Txs that have become invalid in the new state are also discarded.
+// "block" is the new block being committed.
+// "state" is the result of state.AppendBlock("block").
+// Txs that are present in "block" are discarded from mempool.
+// Txs that have become invalid in the new "state" are also discarded.
 func (mem *Mempool) ResetForBlockAndState(block *Block, state *State) {
 	mem.mtx.Lock()
 	defer mem.mtx.Unlock()
diff --git a/merkle/iavl_tree.go b/merkle/iavl_tree.go
index 76cb31443..3a5cad5bd 100644
--- a/merkle/iavl_tree.go
+++ b/merkle/iavl_tree.go
@@ -40,18 +40,24 @@ func NewIAVLTree(keyCodec, valueCodec Codec, cacheSize int, db DB) *IAVLTree {
 // The returned tree and the original tree are goroutine independent.
 // That is, they can each run in their own goroutine.
 func (t *IAVLTree) Copy() Tree {
+	if t.root == nil {
+		return &IAVLTree{
+			keyCodec:   t.keyCodec,
+			valueCodec: t.valueCodec,
+			root:       nil,
+			ndb:        t.ndb,
+		}
+	}
 	if t.ndb != nil && !t.root.persisted {
-		panic("It is unsafe to Copy() an unpersisted tree.")
 		// Saving a tree finalizes all the nodes.
 		// It sets all the hashes recursively,
 		// clears all the leftNode/rightNode values recursively,
 		// and all the .persisted flags get set.
-		// On the other hand, in-memory trees (ndb == nil)
-		// don't mutate
+		panic("It is unsafe to Copy() an unpersisted tree.")
 	} else if t.ndb == nil && t.root.hash == nil {
-		panic("An in-memory IAVLTree must be hashed first")
 		// An in-memory IAVLTree is finalized when the hashes are
 		// calculated.
+		t.root.hashWithCount(t)
 	}
 	return &IAVLTree{
 		keyCodec:   t.keyCodec,
diff --git a/state/state.go b/state/state.go
index f7b8d8719..bfc9b9b55 100644
--- a/state/state.go
+++ b/state/state.go
@@ -314,9 +314,12 @@ func (s *State) releaseValidator(accountId uint64) {
 	}
 }
 
+// "checkStateHash": If false, instead of checking the resulting
+// state.Hash() against block.StateHash, it *sets* the block.StateHash.
+// (used for constructing a new proposal)
 // NOTE: If an error occurs during block execution, state will be left
 // at an invalid state.  Copy the state before calling AppendBlock!
-func (s *State) AppendBlock(b *Block) error {
+func (s *State) AppendBlock(b *Block, checkStateHash bool) error {
 	// Basic block validation.
 	err := b.ValidateBasic(s.Height, s.BlockHash)
 	if err != nil {
@@ -373,15 +376,20 @@ func (s *State) AppendBlock(b *Block) error {
 	// Increment validator AccumPowers
 	s.BondedValidators.IncrementAccum()
 
-	// State hashes should match
-	// XXX include UnbondingValidators.Hash().
-	if !bytes.Equal(s.BondedValidators.Hash(), b.ValidationStateHash) {
-		return Errorf("Invalid ValidationStateHash. Got %X, block says %X",
-			s.BondedValidators.Hash(), b.ValidationStateHash)
-	}
-	if !bytes.Equal(s.AccountDetails.Hash(), b.AccountStateHash) {
-		return Errorf("Invalid AccountStateHash. Got %X, block says %X",
-			s.AccountDetails.Hash(), b.AccountStateHash)
+	// Check or set block.StateHash
+	stateHash := s.Hash()
+	if checkStateHash {
+		// State hash should match
+		if !bytes.Equal(stateHash, b.StateHash) {
+			return Errorf("Invalid state hash. Got %X, block says %X",
+				stateHash, b.StateHash)
+		}
+	} else {
+		// Set the state hash.
+		if b.StateHash != nil {
+			panic("Cannot overwrite block.StateHash")
+		}
+		b.StateHash = stateHash
 	}
 
 	s.Height = b.Height
@@ -401,3 +409,14 @@ func (s *State) GetAccountDetail(accountId uint64) *AccountDetail {
 func (s *State) SetAccountDetail(accDet *AccountDetail) (updated bool) {
 	return s.AccountDetails.Set(accDet.Id, accDet)
 }
+
+// Returns a hash that represents the state data,
+// excluding Height, BlockHash, and CommitTime.
+func (s *State) Hash() []byte {
+	hashables := []merkle.Hashable{
+		s.AccountDetails,
+		s.BondedValidators,
+		s.UnbondingValidators,
+	}
+	return merkle.HashFromHashables(hashables)
+}
diff --git a/state/state_test.go b/state/state_test.go
index 66c43fc5d..0d8b686b1 100644
--- a/state/state_test.go
+++ b/state/state_test.go
@@ -35,32 +35,67 @@ func randGenesisState(numAccounts int, numValidators int) *State {
 		}
 	}
 	s0 := GenesisState(db, time.Now(), accountDetails)
+	s0.Save(time.Now())
 	return s0
 }
 
+func TestCopyState(t *testing.T) {
+	// Generate a state
+	s0 := randGenesisState(10, 5)
+	s0Hash := s0.Hash()
+	if len(s0Hash) == 0 {
+		t.Error("Expected state hash")
+	}
+
+	// Check hash of copy
+	s0Copy := s0.Copy()
+	if !bytes.Equal(s0Hash, s0Copy.Hash()) {
+		t.Error("Expected state copy hash to be the same")
+	}
+
+	// Mutate the original.
+	_, accDet_ := s0.AccountDetails.GetByIndex(0)
+	accDet := accDet_.(*AccountDetail)
+	if accDet == nil {
+		t.Error("Expected state to have an account")
+	}
+	accDet.Balance += 1
+	s0.AccountDetails.Set(accDet.Id, accDet)
+	if bytes.Equal(s0Hash, s0.Hash()) {
+		t.Error("Expected state hash to have changed")
+	}
+	if !bytes.Equal(s0Hash, s0Copy.Hash()) {
+		t.Error("Expected state copy hash to have not changed")
+	}
+}
+
 func TestGenesisSaveLoad(t *testing.T) {
 
 	// Generate a state, save & load it.
 	s0 := randGenesisState(10, 5)
-	// Figure out what the next state hashes should be.
-	s0.BondedValidators.Hash()
-	s0ValsCopy := s0.BondedValidators.Copy()
-	s0ValsCopy.IncrementAccum()
-	nextValidationStateHash := s0ValsCopy.Hash()
-	nextAccountStateHash := s0.AccountDetails.Hash()
 	// Mutate the state to append one empty block.
 	block := &Block{
 		Header: Header{
-			Network:             Config.Network,
-			Height:              1,
-			ValidationStateHash: nextValidationStateHash,
-			AccountStateHash:    nextAccountStateHash,
+			Network:   Config.Network,
+			Height:    1,
+			StateHash: nil,
 		},
 		Data: Data{
 			Txs: []Tx{},
 		},
 	}
-	err := s0.AppendBlock(block)
+	// The second argument to AppendBlock() is false,
+	// which sets Block.Header.StateHash.
+	err := s0.Copy().AppendBlock(block, false)
+	if err != nil {
+		t.Error("Error appending initial block:", err)
+	}
+	if len(block.Header.StateHash) == 0 {
+		t.Error("Expected StateHash but got nothing.")
+	}
+	// Now append the block to s0.
+	// This time we also check the StateHash (as computed above).
+	err = s0.AppendBlock(block, true)
 	if err != nil {
 		t.Error("Error appending initial block:", err)
 	}
@@ -92,13 +127,28 @@ func TestGenesisSaveLoad(t *testing.T) {
 	if !bytes.Equal(s0.BlockHash, s1.BlockHash) {
 		t.Error("BlockHash mismatch")
 	}
-	// Compare BondedValidators
+	// Compare state merkle trees
 	if s0.BondedValidators.Size() != s1.BondedValidators.Size() {
 		t.Error("BondedValidators Size mismatch")
 	}
 	if s0.BondedValidators.TotalVotingPower() != s1.BondedValidators.TotalVotingPower() {
 		t.Error("BondedValidators TotalVotingPower mismatch")
 	}
+	if bytes.Equal(s0.BondedValidators.Hash(), s1.BondedValidators.Hash()) {
+		// The BondedValidators hash should have changed because
+		// each AppendBlock() calls IncrementAccum(),
+		// changing each validator's Accum.
+		t.Error("BondedValidators hash should have changed")
+	}
+	if s0.UnbondingValidators.Size() != s1.UnbondingValidators.Size() {
+		t.Error("UnbondingValidators Size mismatch")
+	}
+	if s0.UnbondingValidators.TotalVotingPower() != s1.UnbondingValidators.TotalVotingPower() {
+		t.Error("UnbondingValidators TotalVotingPower mismatch")
+	}
+	if !bytes.Equal(s0.UnbondingValidators.Hash(), s1.UnbondingValidators.Hash()) {
+		t.Error("UnbondingValidators hash mismatch")
+	}
 	if !bytes.Equal(s0.AccountDetails.Hash(), s1.AccountDetails.Hash()) {
 		t.Error("AccountDetail mismatch")
 	}