diff --git a/client/client.go b/client/client.go
index a8bc8ea8..fd31b21a 100644
--- a/client/client.go
+++ b/client/client.go
@@ -21,7 +21,7 @@ type Client interface {
 	DeliverTxAsync(tx []byte) *ReqRes
 	CheckTxAsync(tx []byte) *ReqRes
 	QueryAsync(tx []byte) *ReqRes
-	ProofAsync(tx []byte) *ReqRes
+	ProofAsync(key []byte, blockHeight int64) *ReqRes
 	CommitAsync() *ReqRes
 
 	FlushSync() error
@@ -31,7 +31,7 @@ type Client interface {
 	DeliverTxSync(tx []byte) (res types.Result)
 	CheckTxSync(tx []byte) (res types.Result)
 	QuerySync(tx []byte) (res types.Result)
-	ProofSync(tx []byte) (res types.Result)
+	ProofSync(key []byte, blockHeight int64) (res types.Result)
 	CommitSync() (res types.Result)
 
 	InitChainAsync(validators []*types.Validator) *ReqRes
diff --git a/client/grpc_client.go b/client/grpc_client.go
index 740623c6..3654c47c 100644
--- a/client/grpc_client.go
+++ b/client/grpc_client.go
@@ -182,8 +182,8 @@ func (cli *grpcClient) QueryAsync(query []byte) *ReqRes {
 	return cli.finishAsyncCall(req, &types.Response{&types.Response_Query{res}})
 }
 
-func (cli *grpcClient) ProofAsync(key []byte) *ReqRes {
-	req := types.ToRequestProof(key)
+func (cli *grpcClient) ProofAsync(key []byte, blockHeight int64) *ReqRes {
+	req := types.ToRequestProof(key, blockHeight)
 	res, err := cli.client.Proof(context.Background(), req.GetProof(), grpc.FailFast(true))
 	if err != nil {
 		cli.StopForError(err)
@@ -310,8 +310,8 @@ func (cli *grpcClient) CheckTxSync(tx []byte) (res types.Result) {
 	return types.Result{Code: resp.Code, Data: resp.Data, Log: resp.Log}
 }
 
-func (cli *grpcClient) ProofSync(key []byte) (res types.Result) {
-	reqres := cli.ProofAsync(key)
+func (cli *grpcClient) ProofSync(key []byte, blockHeight int64) (res types.Result) {
+	reqres := cli.ProofAsync(key, blockHeight)
 	if res := cli.checkErrGetResult(); res.IsErr() {
 		return res
 	}
diff --git a/client/local_client.go b/client/local_client.go
index 8c092cec..bdc59c5a 100644
--- a/client/local_client.go
+++ b/client/local_client.go
@@ -99,12 +99,12 @@ func (app *localClient) QueryAsync(tx []byte) *ReqRes {
 	)
 }
 
-func (app *localClient) ProofAsync(key []byte) *ReqRes {
+func (app *localClient) ProofAsync(key []byte, blockHeight int64) *ReqRes {
 	app.mtx.Lock()
-	res := app.Application.Proof(key)
+	res := app.Application.Proof(key, blockHeight)
 	app.mtx.Unlock()
 	return app.callback(
-		types.ToRequestProof(key),
+		types.ToRequestProof(key, blockHeight),
 		types.ToResponseQuery(res.Code, res.Data, res.Log),
 	)
 }
@@ -202,9 +202,9 @@ func (app *localClient) QuerySync(query []byte) (res types.Result) {
 	return res
 }
 
-func (app *localClient) ProofSync(key []byte) (res types.Result) {
+func (app *localClient) ProofSync(key []byte, blockHeight int64) (res types.Result) {
 	app.mtx.Lock()
-	res = app.Application.Proof(key)
+	res = app.Application.Proof(key, blockHeight)
 	app.mtx.Unlock()
 	return res
 }
diff --git a/client/socket_client.go b/client/socket_client.go
index 252c8777..cacd67bf 100644
--- a/client/socket_client.go
+++ b/client/socket_client.go
@@ -255,8 +255,8 @@ func (cli *socketClient) QueryAsync(query []byte) *ReqRes {
 	return cli.queueRequest(types.ToRequestQuery(query))
 }
 
-func (cli *socketClient) ProofAsync(key []byte) *ReqRes {
-	return cli.queueRequest(types.ToRequestProof(key))
+func (cli *socketClient) ProofAsync(key []byte, blockHeight int64) *ReqRes {
+	return cli.queueRequest(types.ToRequestProof(key, blockHeight))
 }
 
 func (cli *socketClient) CommitAsync() *ReqRes {
@@ -349,8 +349,8 @@ func (cli *socketClient) QuerySync(query []byte) (res types.Result) {
 	return types.Result{Code: resp.Code, Data: resp.Data, Log: resp.Log}
 }
 
-func (cli *socketClient) ProofSync(key []byte) (res types.Result) {
-	reqres := cli.queueRequest(types.ToRequestProof(key))
+func (cli *socketClient) ProofSync(key []byte, blockHeight int64) (res types.Result) {
+	reqres := cli.queueRequest(types.ToRequestProof(key, blockHeight))
 	cli.FlushSync()
 	if err := cli.Error(); err != nil {
 		return types.ErrInternalError.SetLog(err.Error())
diff --git a/cmd/abci-cli/tmsp-cli.go b/cmd/abci-cli/tmsp-cli.go
index d40b510b..0eae4d85 100644
--- a/cmd/abci-cli/tmsp-cli.go
+++ b/cmd/abci-cli/tmsp-cli.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strconv"
 	"strings"
 
 	"github.com/tendermint/abci/client"
@@ -315,12 +316,21 @@ func cmdQuery(c *cli.Context) error {
 // Prove application state
 func cmdProof(c *cli.Context) error {
 	args := c.Args()
-	if len(args) != 1 {
-		return errors.New("Command proof takes 1 argument")
+	if len(args) < 1 {
+		return errors.New("Command proof takes 1 or 2 arguments")
 	}
-	keyBytes := stringOrHexToBytes(c.Args()[0])
-	res := client.ProofSync(keyBytes)
-	printResponse(c, res, string(res.Data), true)
+	keyBytes, err := stringOrHexToBytes(c.Args()[0])
+	if err != nil {
+		return err
+	}
+
+	var height int64
+	if len(args) == 2 {
+		height, _ = strconv.ParseInt(args[1], 10, 0)
+	}
+	res := client.ProofSync(keyBytes, height)
+	rsp := newResponse(res, string(res.Data), true)
+	printResponse(c, rsp)
 	return nil
 }
 
diff --git a/example/dummy/dummy.go b/example/dummy/dummy.go
index ebce1099..fcf067dc 100644
--- a/example/dummy/dummy.go
+++ b/example/dummy/dummy.go
@@ -2,6 +2,7 @@ package dummy
 
 import (
 	"encoding/hex"
+	"fmt"
 	"strings"
 
 	"github.com/tendermint/abci/types"
@@ -54,7 +55,15 @@ func (app *DummyApplication) Query(query []byte) types.Result {
 }
 
 func (app *DummyApplication) Proof(key []byte, blockHeight int64) types.Result {
-	return types.NewResultOK(nil, Fmt("TODO: support proof!"))
+	if blockHeight != 0 {
+		return types.ErrUnknownRequest
+	}
+	proof, exists := app.state.Proof(key)
+	if !exists {
+		fmt.Println("Didn't find nothing")
+		return types.NewResultOK(nil, "")
+	}
+	return types.NewResultOK(proof, "Found the key")
 }
 
 type QueryResult struct {
diff --git a/example/dummy/dummy_test.go b/example/dummy/dummy_test.go
index d2294347..8cd999c0 100644
--- a/example/dummy/dummy_test.go
+++ b/example/dummy/dummy_test.go
@@ -8,6 +8,7 @@ import (
 
 	. "github.com/tendermint/go-common"
 	"github.com/tendermint/go-crypto"
+	merkle "github.com/tendermint/go-merkle"
 	"github.com/tendermint/go-wire"
 	"github.com/tendermint/abci/types"
 )
@@ -34,6 +35,27 @@ func testDummy(t *testing.T, dummy types.Application, tx []byte, key, value stri
 		t.Fatalf("Got %s, expected %s", q.Value, value)
 	}
 
+	rp := dummy.Proof([]byte(key), 0)
+	if rp.IsErr() {
+		t.Fatal(rp)
+	}
+
+	p, err := merkle.LoadProof(rp.Data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !p.Valid() {
+		t.Fatal("Invalid proof")
+	}
+
+	if !bytes.Equal([]byte(key), p.Key()) {
+		t.Fatalf("Invalid key: %s", p.Key())
+	}
+
+	if !bytes.Equal([]byte(value), p.Value()) {
+		t.Fatalf("Invalid key: %s", p.Value())
+	}
 }
 
 func TestDummyKV(t *testing.T) {
diff --git a/example/dummy/persistent_dummy.go b/example/dummy/persistent_dummy.go
index 70c0e667..3a1ce3b5 100644
--- a/example/dummy/persistent_dummy.go
+++ b/example/dummy/persistent_dummy.go
@@ -94,7 +94,7 @@ func (app *PersistentDummyApplication) Query(query []byte) types.Result {
 }
 
 func (app *PersistentDummyApplication) Proof(key []byte, blockHeight int64) types.Result {
-	return types.NewResultOK(nil, Fmt("TODO: support proof!"))
+	return app.app.Proof(key, blockHeight)
 }
 
 // Save the validators in the merkle tree