diff --git a/app/app.go b/app/app.go
index 6f107f2b7..12fea597f 100644
--- a/app/app.go
+++ b/app/app.go
@@ -8,12 +8,13 @@ import (
 	"github.com/golang/protobuf/proto"
 	"github.com/pkg/errors"
 	abci "github.com/tendermint/abci/types"
+	cmn "github.com/tendermint/tmlibs/common"
 	"github.com/tendermint/tmlibs/log"
 
 	"github.com/cosmos/cosmos-sdk/types"
 )
 
-var mainHeaderKey = "header"
+var mainHeaderKey = []byte("header")
 
 // App - The ABCI application
 type App struct {
@@ -23,16 +24,16 @@ type App struct {
 	name string
 
 	// Main (uncached) state
-	store types.CommitMultiStore
+	ms types.CommitMultiStore
 
-	// CheckTx state, a cache-wrap of store.
-	storeCheck types.CacheMultiStore
+	// CheckTx state, a cache-wrap of `.ms`.
+	msCheck types.CacheMultiStore
 
-	// DeliverTx state, a cache-wrap of store.
-	storeDeliver types.CacheMultiStore
+	// DeliverTx state, a cache-wrap of `.ms`.
+	msDeliver types.CacheMultiStore
 
 	// Current block header
-	header *abci.Header
+	header abci.Header
 
 	// Handler for CheckTx and DeliverTx.
 	handler types.Handler
@@ -54,8 +55,8 @@ func (app *App) Name() string {
 	return app.name
 }
 
-func (app *App) SetCommitStore(store types.CommitMultiStore) {
-	app.store = store
+func (app *App) SetCommitMultiStore(ms types.CommitMultiStore) {
+	app.ms = ms
 }
 
 func (app *App) SetHandler(handler types.Handler) {
@@ -63,26 +64,33 @@ func (app *App) SetHandler(handler types.Handler) {
 }
 
 func (app *App) LoadLatestVersion() error {
-	store := app.store
-	store.LoadLatestVersion()
+	app.ms.LoadLatestVersion()
 	return app.initFromStore()
 }
 
 func (app *App) LoadVersion(version int64) error {
-	store := app.store
-	store.LoadVersion(version)
+	app.ms.LoadVersion(version)
 	return app.initFromStore()
 }
 
-// Initializes the remaining logic from app.store.
+// The last CommitID of the multistore.
+func (app *App) LastCommitID() types.CommitID {
+	return app.ms.LastCommitID()
+}
+
+// The last commited block height.
+func (app *App) LastBlockHeight() int64 {
+	return app.ms.LastCommitID().Version
+}
+
+// Initializes the remaining logic from app.ms.
 func (app *App) initFromStore() error {
-	store := app.store
-	lastCommitID := store.LastCommitID()
-	main := store.GetKVStore("main")
-	header := (*abci.Header)(nil)
+	lastCommitID := app.ms.LastCommitID()
+	main := app.ms.GetKVStore("main")
+	header := abci.Header{}
 
 	// Main store should exist.
-	if store.GetKVStore("main") == nil {
+	if app.ms.GetKVStore("main") == nil {
 		return errors.New("App expects MultiStore with 'main' KVStore")
 	}
 
@@ -93,7 +101,7 @@ func (app *App) initFromStore() error {
 			errStr := fmt.Sprintf("Version > 0 but missing key %s", mainHeaderKey)
 			return errors.New(errStr)
 		}
-		err := proto.Unmarshal(headerBytes, header)
+		err := proto.Unmarshal(headerBytes, &header)
 		if err != nil {
 			return errors.Wrap(err, "Failed to parse Header")
 		}
@@ -106,8 +114,8 @@ func (app *App) initFromStore() error {
 
 	// Set App state.
 	app.header = header
-	app.storeCheck = nil
-	app.storeDeliver = nil
+	app.msCheck = nil
+	app.msDeliver = nil
 	app.valUpdates = nil
 
 	return nil
@@ -118,7 +126,7 @@ func (app *App) initFromStore() error {
 // Implements ABCI
 func (app *App) Info(req abci.RequestInfo) abci.ResponseInfo {
 
-	lastCommitID := app.store.LastCommitID()
+	lastCommitID := app.ms.LastCommitID()
 
 	return abci.ResponseInfo{
 		Data:             app.name,
@@ -148,8 +156,8 @@ func (app *App) Query(req abci.RequestQuery) (res abci.ResponseQuery) {
 // Implements ABCI
 func (app *App) BeginBlock(req abci.RequestBeginBlock) (res abci.ResponseBeginBlock) {
 	app.header = req.Header
-	app.storeDeliver = app.store.CacheMultiStore()
-	app.storeCheck = app.store.CacheMultiStore()
+	app.msDeliver = app.ms.CacheMultiStore()
+	app.msCheck = app.ms.CacheMultiStore()
 	return
 }
 
@@ -159,20 +167,22 @@ func (app *App) CheckTx(txBytes []byte) (res abci.ResponseCheckTx) {
 	// Initialize arguments to Handler.
 	var isCheckTx = true
 	var ctx = types.NewContext(app.header, isCheckTx, txBytes)
-	var store = app.store
-	var tx Tx = nil // nil until a decorator parses one.
+	var tx types.Tx = nil // nil until a decorator parses one.
 
 	// Run the handler.
-	var result = app.handler(ctx, app.store, tx)
+	var result = app.handler(ctx, app.ms, tx)
 
 	// Tell the blockchain engine (i.e. Tendermint).
-	return abci.ResponseDeliverTx{
+	return abci.ResponseCheckTx{
 		Code:      result.Code,
 		Data:      result.Data,
 		Log:       result.Log,
-		Gas:       result.Gas,
-		FeeDenom:  result.FeeDenom,
-		FeeAmount: result.FeeAmount,
+		GasWanted: result.GasWanted,
+		Fee: cmn.KI64Pair{
+			[]byte(result.FeeDenom),
+			result.FeeAmount,
+		},
+		Tags: result.Tags,
 	}
 }
 
@@ -182,15 +192,14 @@ func (app *App) DeliverTx(txBytes []byte) (res abci.ResponseDeliverTx) {
 	// Initialize arguments to Handler.
 	var isCheckTx = false
 	var ctx = types.NewContext(app.header, isCheckTx, txBytes)
-	var store = app.store
-	var tx Tx = nil // nil until a decorator parses one.
+	var tx types.Tx = nil // nil until a decorator parses one.
 
 	// Run the handler.
-	var result = app.handler(ctx, app.store, tx)
+	var result = app.handler(ctx, app.ms, tx)
 
 	// After-handler hooks.
-	if result.Code == abci.CodeType_OK {
-		app.valUpdates = append(app.valUpdates, result.ValUpdate)
+	if result.Code == abci.CodeTypeOK {
+		app.valUpdates = append(app.valUpdates, result.ValidatorUpdates...)
 	} else {
 		// Even though the Code is not OK, there will be some side effects,
 		// like those caused by fee deductions or sequence incrementations.
@@ -198,10 +207,12 @@ func (app *App) DeliverTx(txBytes []byte) (res abci.ResponseDeliverTx) {
 
 	// Tell the blockchain engine (i.e. Tendermint).
 	return abci.ResponseDeliverTx{
-		Code: result.Code,
-		Data: result.Data,
-		Log:  result.Log,
-		Tags: result.Tags,
+		Code:      result.Code,
+		Data:      result.Data,
+		Log:       result.Log,
+		GasWanted: result.GasWanted,
+		GasUsed:   result.GasUsed,
+		Tags:      result.Tags,
 	}
 }
 
@@ -214,12 +225,14 @@ func (app *App) EndBlock(req abci.RequestEndBlock) (res abci.ResponseEndBlock) {
 
 // Implements ABCI
 func (app *App) Commit() (res abci.ResponseCommit) {
-	app.storeDeliver.Write()
-	commitID := app.store.Commit()
+	app.msDeliver.Write()
+	commitID := app.ms.Commit()
 	app.logger.Debug("Commit synced",
 		"commit", commitID,
 	)
-	return abci.NewResultOK(hash, "")
+	return abci.ResponseCommit{
+		Data: commitID.Hash,
+	}
 }
 
 //----------------------------------------
diff --git a/app/app_test.go b/app/app_test.go
index 3ac550357..befb25145 100644
--- a/app/app_test.go
+++ b/app/app_test.go
@@ -7,12 +7,13 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 
-	sdk "github.com/cosmos/cosmos-sdk"
+	"github.com/cosmos/cosmos-sdk/store"
+	"github.com/cosmos/cosmos-sdk/types"
 	abci "github.com/tendermint/abci/types"
 	"github.com/tendermint/go-crypto"
 	cmn "github.com/tendermint/tmlibs/common"
+	dbm "github.com/tendermint/tmlibs/db"
 )
 
 func TestBasic(t *testing.T) {
@@ -24,31 +25,33 @@ func TestBasic(t *testing.T) {
 	}
 
 	// Create app.
-	app := sdk.NewApp(t.Name())
-	app.SetStore(mockMultiStore())
-	app.SetHandler(func(ctx Context, store MultiStore, tx Tx) Result {
+	app := NewApp(t.Name())
+	app.SetCommitMultiStore(newCommitMultiStore())
+	app.SetHandler(func(ctx types.Context, store types.MultiStore, tx types.Tx) types.Result {
 
 		// This could be a decorator.
-		fromJSON(ctx.TxBytes(), &tx)
+		var ttx testTx
+		fromJSON(ctx.TxBytes(), &ttx)
 
-		fmt.Println(">>", tx)
+		// XXX
+		return types.Result{}
 	})
 
 	// Load latest state, which should be empty.
 	err := app.LoadLatestVersion()
 	assert.Nil(t, err)
-	assert.Equal(t, app.NextVersion(), 1)
+	assert.Equal(t, app.LastBlockHeight(), int64(0))
 
 	// Create the validators
 	var numVals = 3
-	var valSet = make([]*abci.Validator, numVals)
+	var valSet = make([]abci.Validator, numVals)
 	for i := 0; i < numVals; i++ {
 		valSet[i] = makeVal(secret(i))
 	}
 
 	// Initialize the chain
 	app.InitChain(abci.RequestInitChain{
-		Validators: valset,
+		Validators: valSet,
 	})
 
 	// Simulate the start of a block.
@@ -62,24 +65,26 @@ func TestBasic(t *testing.T) {
 		}
 		txBytes := toJSON(tx)
 		res := app.DeliverTx(txBytes)
-		require.True(res.IsOK(), "%#v", res)
+		assert.True(t, res.IsOK(), "%#v", res)
 	}
 
 	// Simulate the end of a block.
 	// Get the summary of validator updates.
-	res := app.EndBlock(app.height)
+	res := app.EndBlock(abci.RequestEndBlock{})
 	valUpdates := res.ValidatorUpdates
 
 	// Assert that validator updates are correct.
 	for _, val := range valSet {
+		// Sanity
+		assert.NotEqual(t, len(val.PubKey), 0)
 
 		// Find matching update and splice it out.
 		for j := 0; j < len(valUpdates); {
-			assert.NotEqual(len(valUpdates.PubKey), 0)
+			valUpdate := valUpdates[j]
 
 			// Matched.
 			if bytes.Equal(valUpdate.PubKey, val.PubKey) {
-				assert.Equal(valUpdate.NewPower, val.Power+1)
+				assert.Equal(t, valUpdate.Power, val.Power+1)
 				if j < len(valUpdates)-1 {
 					// Splice it out.
 					valUpdates = append(valUpdates[:j], valUpdates[j+1:]...)
@@ -100,9 +105,9 @@ func randPower() int64 {
 	return cmn.RandInt64()
 }
 
-func makeVal(secret string) *abci.Validator {
-	return &abci.Validator{
-		PubKey: makePubKey(string).Bytes(),
+func makeVal(secret string) abci.Validator {
+	return abci.Validator{
+		PubKey: makePubKey(secret).Bytes(),
 		Power:  randPower(),
 	}
 }
@@ -112,29 +117,43 @@ func makePubKey(secret string) crypto.PubKey {
 }
 
 func makePrivKey(secret string) crypto.PrivKey {
-	return crypto.GenPrivKeyEd25519FromSecret([]byte(id))
+	privKey := crypto.GenPrivKeyEd25519FromSecret([]byte(secret))
+	return privKey.Wrap()
 }
 
-func secret(index int) []byte {
-	return []byte(fmt.Sprintf("secret%d", index))
+func secret(index int) string {
+	return fmt.Sprintf("secret%d", index)
 }
 
-func copyVal(val *abci.Validator) *abci.Validator {
-	val2 := *val
-	return &val2
+func copyVal(val abci.Validator) abci.Validator {
+	// val2 := *val
+	// return &val2
+	return val
 }
 
 func toJSON(o interface{}) []byte {
-	bytes, err := json.Marshal(o)
+	bz, err := json.Marshal(o)
 	if err != nil {
 		panic(err)
 	}
-	return bytes
+	// fmt.Println(">> toJSON:", string(bz))
+	return bz
 }
 
-func fromJSON(bytes []byte, ptr interface{}) {
-	err := json.Unmarshal(bytes, ptr)
+func fromJSON(bz []byte, ptr interface{}) {
+	// fmt.Println(">> fromJSON:", string(bz))
+	err := json.Unmarshal(bz, ptr)
 	if err != nil {
 		panic(err)
 	}
 }
+
+// Creates a sample CommitMultiStore
+func newCommitMultiStore() types.CommitMultiStore {
+	dbMain := dbm.NewMemDB()
+	dbXtra := dbm.NewMemDB()
+	ms := store.NewMultiStore(dbMain) // Also store rootMultiStore metadata here (it shouldn't clash)
+	ms.SetSubstoreLoader("main", store.NewIAVLStoreLoader(dbMain, 0, 0))
+	ms.SetSubstoreLoader("xtra", store.NewIAVLStoreLoader(dbXtra, 0, 0))
+	return ms
+}
diff --git a/glide.lock b/glide.lock
index bb1cdd8cb..b2a9eef33 100644
--- a/glide.lock
+++ b/glide.lock
@@ -112,7 +112,7 @@ imports:
   - leveldb/table
   - leveldb/util
 - name: github.com/tendermint/abci
-  version: 4274f8eca234325d2fbc9a434f9bc099bd2c40d4
+  version: 8f87efd7f86c2bae9df69aff69d715593d5d79f7
   subpackages:
   - client
   - example/dummy
@@ -174,7 +174,7 @@ imports:
   - types
   - version
 - name: github.com/tendermint/tmlibs
-  version: b70ae4919befb6ae3e5cb40ae8174e122e771d08
+  version: b25df389db3c98f4b964bd39511c199f02d07715
   subpackages:
   - autofile
   - cli
diff --git a/store/iavlstore.go b/store/iavlstore.go
index 0c333ab92..c349358c7 100644
--- a/store/iavlstore.go
+++ b/store/iavlstore.go
@@ -48,6 +48,7 @@ type iavlStore struct {
 	tree *iavl.VersionedTree
 
 	// How many old versions we hold onto.
+	// A value of 0 means keep all history.
 	numHistory int64
 }
 
@@ -71,7 +72,7 @@ func (st *iavlStore) Commit() CommitID {
 	}
 
 	// Release an old version of history
-	if st.numHistory < st.tree.Version64() {
+	if st.numHistory > 0 && (st.numHistory < st.tree.Version64()) {
 		toRelease := version - st.numHistory
 		st.tree.DeleteVersion(toRelease)
 	}
diff --git a/store/rootmultistore.go b/store/rootmultistore.go
index 21b399946..7508d3c05 100644
--- a/store/rootmultistore.go
+++ b/store/rootmultistore.go
@@ -26,6 +26,8 @@ type rootMultiStore struct {
 	substores    map[string]CommitStore
 }
 
+var _ CommitMultiStore = (*rootMultiStore)(nil)
+
 func NewMultiStore(db dbm.DB) *rootMultiStore {
 	return &rootMultiStore{
 		db:           db,
@@ -35,6 +37,7 @@ func NewMultiStore(db dbm.DB) *rootMultiStore {
 	}
 }
 
+// Implements CommitMultiStore.
 func (rs *rootMultiStore) SetSubstoreLoader(name string, loader CommitStoreLoader) {
 	if _, ok := rs.storeLoaders[name]; ok {
 		panic(fmt.Sprintf("rootMultiStore duplicate substore name " + name))
@@ -42,17 +45,18 @@ func (rs *rootMultiStore) SetSubstoreLoader(name string, loader CommitStoreLoade
 	rs.storeLoaders[name] = loader
 }
 
-// Call once after all calls to SetSubstoreLoader are complete.
+// Implements CommitMultiStore.
+func (rs *rootMultiStore) GetSubstore(name string) CommitStore {
+	return rs.substores[name]
+}
+
+// Implements CommitMultiStore.
 func (rs *rootMultiStore) LoadLatestVersion() error {
 	ver := getLatestVersion(rs.db)
 	return rs.LoadVersion(ver)
 }
 
-// NOTE: Returns 0 unless LoadVersion() or LoadLatestVersion() is called.
-func (rs *rootMultiStore) NextVersion() int64 {
-	return rs.nextVersion
-}
-
+// Implements CommitMultiStore.
 func (rs *rootMultiStore) LoadVersion(ver int64) error {
 
 	// Special logic for version 0
@@ -106,10 +110,13 @@ func (rs *rootMultiStore) LoadVersion(ver int64) error {
 	return nil
 }
 
-// Implements CommitStore
+//----------------------------------------
+// +CommitStore
+
+// Implements CommitStore.
 func (rs *rootMultiStore) Commit() CommitID {
 
-	// Commit substores
+	// Commit substores.
 	version := rs.nextVersion
 	state := commitSubstores(version, rs.substores)
 
@@ -129,27 +136,36 @@ func (rs *rootMultiStore) Commit() CommitID {
 	return commitID
 }
 
-// Implements CommitStore
+// Implements CommitStore.
 func (rs *rootMultiStore) CacheWrap() CacheWrap {
 	return rs.CacheMultiStore().(CacheWrap)
 }
 
-// Get the last committed CommitID
+//----------------------------------------
+// +MultiStore
+
+// Implements MultiStore.
 func (rs *rootMultiStore) LastCommitID() CommitID {
 	return rs.lastCommitID
 }
 
-// Implements MultiStore
+// Implements MultiStore.
+// NOTE: Returns 0 unless LoadVersion() or LoadLatestVersion() is called.
+func (rs *rootMultiStore) NextVersion() int64 {
+	return rs.nextVersion
+}
+
+// Implements MultiStore.
 func (rs *rootMultiStore) CacheMultiStore() CacheMultiStore {
 	return newCacheMultiStoreFromRMS(rs)
 }
 
-// Implements MultiStore
-func (rs *rootMultiStore) GetCommitStore(name string) CommitStore {
+// Implements MultiStore.
+func (rs *rootMultiStore) GetStore(name string) interface{} {
 	return rs.substores[name]
 }
 
-// Implements MultiStore
+// Implements MultiStore.
 func (rs *rootMultiStore) GetKVStore(name string) KVStore {
 	return rs.substores[name].(KVStore)
 }
@@ -208,6 +224,7 @@ func (sc substoreCore) Hash() []byte {
 }
 
 //----------------------------------------
+// Misc.
 
 func getLatestVersion(db dbm.DB) int64 {
 	var latest int64
diff --git a/types/result.go b/types/result.go
index d56e5ceb1..3ce194c25 100644
--- a/types/result.go
+++ b/types/result.go
@@ -17,8 +17,8 @@ type Result struct {
 	// Log is just debug information. NOTE: nondeterministic.
 	Log string
 
-	// GasAllocated is the maximum units of work we allow this tx to perform.
-	GasAllocated int64
+	// GasWanted is the maximum units of work we allow this tx to perform.
+	GasWanted int64
 
 	// GasUsed is the amount of gas actually consumed. NOTE: not used.
 	GasUsed int64
@@ -28,7 +28,7 @@ type Result struct {
 	FeeDenom  string
 
 	// Changes to the validator set.
-	ValSetDiff []abci.Validator
+	ValidatorUpdates []abci.Validator
 
 	// Tags are used for transaction indexing and pubsub.
 	Tags []cmn.KVPair
diff --git a/types/store.go b/types/store.go
index 73a0e848a..adeba6f44 100644
--- a/types/store.go
+++ b/types/store.go
@@ -26,7 +26,7 @@ type MultiStore interface {
 	// call CacheMultiStore.Write().
 	CacheMultiStore() CacheMultiStore
 
-	// Convenience
+	// Convenience for fetching substores.
 	GetStore(name string) interface{}
 	GetKVStore(name string) KVStore
 }
@@ -56,7 +56,11 @@ type CommitMultiStore interface {
 	// Add a substore loader.
 	SetSubstoreLoader(name string, loader CommitStoreLoader)
 
+	// Gets the substore, which is a CommitSubstore.
+	GetSubstore(name string) CommitStore
+
 	// Load the latest persisted version.
+	// Called once after all calls to SetSubstoreLoader are complete.
 	LoadLatestVersion() error
 
 	// Load a specific persisted version.  When you load an old version, or