diff --git a/call.go b/call.go
index 2318bf0e9..b9da0b2ae 100644
--- a/call.go
+++ b/call.go
@@ -83,7 +83,7 @@ func sendRPC(ctx context.Context, callHdr *transport.CallHdr, t transport.Client
 	// TODO(zhaoq): Support compression.
 	outBuf, err := encode(args, compressionNone)
 	if err != nil {
-		return nil, transport.StreamErrorf(codes.Internal, "%v", err)
+		return nil, transport.StreamErrorf(codes.Internal, "grpc: %v", err)
 	}
 	err = t.Write(stream, outBuf, opts)
 	if err != nil {
diff --git a/clientconn.go b/clientconn.go
index c2df3e7be..eb546f942 100644
--- a/clientconn.go
+++ b/clientconn.go
@@ -34,7 +34,7 @@
 package grpc
 
 import (
-	"fmt"
+	"errors"
 	"sync"
 	"time"
 
@@ -43,6 +43,14 @@ import (
 	"google.golang.org/grpc/transport"
 )
 
+var (
+	// ErrUnspecTarget indicates that the target address is unspecified.
+	ErrUnspecTarget = errors.New("grpc: target is unspecified")
+	// ErrClosingChan indicates that the operation is illegal because the session
+	// is closing.
+	ErrClosingChan = errors.New("grpc: the channel is closing")
+)
+
 type dialOptions struct {
 	protocol    string
 	authOptions []credentials.Credentials
@@ -73,7 +81,7 @@ func WithPerRPCCredentials(creds credentials.Credentials) DialOption {
 // for connection to complete.
 func Dial(target string, opts ...DialOption) (*ClientConn, error) {
 	if target == "" {
-		return nil, fmt.Errorf("rpc.Dial: target is empty")
+		return nil, ErrUnspecTarget
 	}
 	cc := &ClientConn{
 		target: target,
@@ -119,7 +127,7 @@ func (cc *ClientConn) resetTransport(closeTransport bool) error {
 		cc.transportSeq = 0
 		if cc.closing {
 			cc.mu.Unlock()
-			return fmt.Errorf("rpc.ClientConn.resetTransport: the channel is closing")
+			return ErrClosingChan
 		}
 		cc.mu.Unlock()
 		if closeTransport {
@@ -174,7 +182,7 @@ func (cc *ClientConn) wait(ctx context.Context, ts int) (transport.ClientTranspo
 		switch {
 		case cc.closing:
 			cc.mu.Unlock()
-			return nil, 0, fmt.Errorf("ClientConn is closing")
+			return nil, 0, ErrClosingChan
 		case ts < cc.transportSeq:
 			// Worked on a dying transport. Try the new one immediately.
 			defer cc.mu.Unlock()
diff --git a/credentials/credentials.go b/credentials/credentials.go
index 13cf7857d..7a464aa98 100644
--- a/credentials/credentials.go
+++ b/credentials/credentials.go
@@ -235,4 +235,3 @@ func NewServiceAccountFromFile(keyFile string, scope ...string) (Credentials, er
 	}
 	return NewServiceAccountFromKey(jsonKey, scope...)
 }
-
diff --git a/interop/client/client.go b/interop/client/client.go
index 655084d3b..527398667 100644
--- a/interop/client/client.go
+++ b/interop/client/client.go
@@ -193,7 +193,7 @@ func doPingPong(tc testpb.TestServiceClient) {
 	var index int
 	for index < len(reqSizes) {
 		respParam := []*testpb.ResponseParameters{
-			&testpb.ResponseParameters{
+			{
 				Size: proto.Int32(int32(respSizes[index])),
 			},
 		}
diff --git a/rpc_util.go b/rpc_util.go
index fa6e8b06f..f7b7ff4c1 100644
--- a/rpc_util.go
+++ b/rpc_util.go
@@ -165,10 +165,10 @@ func recvProto(p *parser, m proto.Message) error {
 	switch pf {
 	case compressionNone:
 		if err := proto.Unmarshal(d, m); err != nil {
-			return Errorf(codes.Internal, "%v", err)
+			return Errorf(codes.Internal, "grpc: %v", err)
 		}
 	default:
-		return Errorf(codes.Internal, "compression is not supported yet.")
+		return Errorf(codes.Internal, "gprc: compression is not supported yet.")
 	}
 	return nil
 }
@@ -219,7 +219,7 @@ func toRPCErr(err error) error {
 			desc: e.Desc,
 		}
 	}
-	return Errorf(codes.Unknown, "failed to convert %v to rpcErr", err)
+	return Errorf(codes.Unknown, "grpc: failed to convert %v to rpcErr", err)
 }
 
 // convertCode converts a standard Go error into its canonical code. Note that
diff --git a/server.go b/server.go
index 02b478b8d..570dc8106 100644
--- a/server.go
+++ b/server.go
@@ -34,6 +34,7 @@
 package grpc
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -145,6 +146,12 @@ func (s *Server) RegisterService(sd *ServiceDesc, ss interface{}) {
 	s.m[sd.ServiceName] = srv
 }
 
+var (
+	// ErrServerStopped indicates that the operation is now illegal because of
+	// the server being stopped.
+	ErrServerStopped = errors.New("grpc: the server has been stopped")
+)
+
 // Serve accepts incoming connections on the listener lis, creating a new
 // ServerTransport and service goroutine for each. The service goroutines
 // read gRPC request and then call the registered handlers to reply to them.
@@ -153,7 +160,7 @@ func (s *Server) Serve(lis net.Listener) error {
 	s.mu.Lock()
 	if s.lis == nil {
 		s.mu.Unlock()
-		return fmt.Errorf("the server has been stopped")
+		return ErrServerStopped
 	}
 	s.lis[lis] = true
 	s.mu.Unlock()
@@ -340,7 +347,7 @@ func SendHeader(ctx context.Context, md metadata.MD) error {
 	}
 	stream, ok := transport.StreamFromContext(ctx)
 	if !ok {
-		return fmt.Errorf("rpc: failed to fetch the stream from the context %v", ctx)
+		return fmt.Errorf("grpc: failed to fetch the stream from the context %v", ctx)
 	}
 	t := stream.ServerTransport()
 	if t == nil {
@@ -358,7 +365,7 @@ func SetTrailer(ctx context.Context, md metadata.MD) error {
 	}
 	stream, ok := transport.StreamFromContext(ctx)
 	if !ok {
-		return fmt.Errorf("rpc: failed to fetch the stream from the context %v", ctx)
+		return fmt.Errorf("grpc: failed to fetch the stream from the context %v", ctx)
 	}
 	return stream.SetTrailer(md)
 }
diff --git a/stream.go b/stream.go
index 2e5cbcb20..c0266eeb7 100644
--- a/stream.go
+++ b/stream.go
@@ -153,7 +153,7 @@ func (cs *clientStream) SendProto(m proto.Message) (err error) {
 	}()
 	out, err := encode(m, compressionNone)
 	if err != nil {
-		return transport.StreamErrorf(codes.Internal, "%v", err)
+		return transport.StreamErrorf(codes.Internal, "grpc: %v", err)
 	}
 	return cs.t.Write(cs.s, out, &transport.Options{Last: false})
 }
@@ -167,7 +167,7 @@ func (cs *clientStream) RecvProto(m proto.Message) (err error) {
 		// Special handling for client streaming rpc.
 		if err = recvProto(cs.p, m); err != io.EOF {
 			cs.t.CloseStream(cs.s, err)
-			return fmt.Errorf("gRPC client streaming protocol violation: %v, want <EOF>", err)
+			return fmt.Errorf("grpc: client streaming protocol violation: %v, want <EOF>", err)
 		}
 	}
 	if _, ok := err.(transport.ConnectionError); !ok {
@@ -235,7 +235,7 @@ func (ss *serverStream) SetTrailer(md metadata.MD) {
 func (ss *serverStream) SendProto(m proto.Message) error {
 	out, err := encode(m, compressionNone)
 	if err != nil {
-		err = transport.StreamErrorf(codes.Internal, "%v", err)
+		err = transport.StreamErrorf(codes.Internal, "grpc: %v", err)
 		return err
 	}
 	return ss.t.Write(ss.s, out, &transport.Options{Last: false})
diff --git a/test/end2end_test.go b/test/end2end_test.go
index 23b55d8a8..eb4c18866 100644
--- a/test/end2end_test.go
+++ b/test/end2end_test.go
@@ -388,7 +388,7 @@ func TestPingPong(t *testing.T) {
 	var index int
 	for index < len(reqSizes) {
 		respParam := []*testpb.ResponseParameters{
-			&testpb.ResponseParameters{
+			{
 				Size: proto.Int32(int32(respSizes[index])),
 			},
 		}
@@ -443,7 +443,7 @@ func TestMetadataStreamingRPC(t *testing.T) {
 		var index int
 		for index < len(reqSizes) {
 			respParam := []*testpb.ResponseParameters{
-				&testpb.ResponseParameters{
+				{
 					Size: proto.Int32(int32(respSizes[index])),
 				},
 			}
diff --git a/transport/http2_client_transport.go b/transport/http2_client_transport.go
index 9fd6283b0..a5232b50a 100644
--- a/transport/http2_client_transport.go
+++ b/transport/http2_client_transport.go
@@ -117,7 +117,7 @@ func newHTTP2Client(addr string, authOpts []credentials.Credentials) (_ ClientTr
 		conn, connErr = net.Dial("tcp", addr)
 	}
 	if connErr != nil {
-		return nil, ConnectionErrorf("%v", connErr)
+		return nil, ConnectionErrorf("grpc/transport: %v", connErr)
 	}
 	defer func() {
 		if err != nil {
@@ -127,14 +127,14 @@ func newHTTP2Client(addr string, authOpts []credentials.Credentials) (_ ClientTr
 	// Send connection preface to server.
 	n, err := conn.Write(clientPreface)
 	if err != nil {
-		return nil, ConnectionErrorf("%v", err)
+		return nil, ConnectionErrorf("grpc/transport: %v", err)
 	}
 	if n != len(clientPreface) {
-		return nil, ConnectionErrorf("Wrting client preface, wrote %d bytes; want %d", n, len(clientPreface))
+		return nil, ConnectionErrorf("grpc/transport: preface mismatch, wrote %d bytes; want %d", n, len(clientPreface))
 	}
 	framer := http2.NewFramer(conn, conn)
 	if err := framer.WriteSettings(); err != nil {
-		return nil, ConnectionErrorf("%v", err)
+		return nil, ConnectionErrorf("grpc/transport: %v", err)
 	}
 	var buf bytes.Buffer
 	t := &http2Client{
@@ -225,7 +225,7 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 		default:
 		}
 		if err != nil {
-			return nil, StreamErrorf(codes.InvalidArgument, "%v", err)
+			return nil, StreamErrorf(codes.InvalidArgument, "grpc/transport: %v", err)
 		}
 		for k, v := range m {
 			t.hEnc.WriteField(hpack.HeaderField{Name: k, Value: v})
@@ -265,7 +265,7 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 		}
 		if err != nil {
 			t.notifyError()
-			return nil, ConnectionErrorf("%v", err)
+			return nil, ConnectionErrorf("grpc/transport: %v", err)
 		}
 	}
 	s := t.newStream(ctx, callHdr)
@@ -276,7 +276,7 @@ func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Strea
 	}
 	if uint32(len(t.activeStreams)) >= t.maxStreams {
 		t.mu.Unlock()
-		return nil, StreamErrorf(codes.Unavailable, "failed to create new stream because the limit has been reached.")
+		return nil, StreamErrorf(codes.Unavailable, "grpc/transport: failed to create new stream because the limit has been reached.")
 	}
 	t.activeStreams[s.id] = s
 	t.mu.Unlock()
@@ -391,7 +391,7 @@ func (t *http2Client) Write(s *Stream, data []byte, opts *Options) error {
 		// invoked.
 		if err := t.framer.WriteData(s.id, endStream, p); err != nil {
 			t.notifyError()
-			return ConnectionErrorf("%v", err)
+			return ConnectionErrorf("grpc/transport: %v", err)
 		}
 		t.writableChan <- 0
 		if r.Len() == 0 {
diff --git a/transport/http2_server_transport.go b/transport/http2_server_transport.go
index c95597067..39cd0a629 100644
--- a/transport/http2_server_transport.go
+++ b/transport/http2_server_transport.go
@@ -35,7 +35,7 @@ package transport
 
 import (
 	"bytes"
-	"fmt"
+	"errors"
 	"io"
 	"log"
 	"math"
@@ -45,11 +45,15 @@ import (
 
 	"github.com/bradfitz/http2"
 	"github.com/bradfitz/http2/hpack"
+	"golang.org/x/net/context"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/metadata"
-	"golang.org/x/net/context"
 )
 
+// ErrIllegalHeaderWrite indicates that setting header is illegal because of
+// the stream's state.
+var ErrIllegalHeaderWrite = errors.New("grpc/transport: the stream is done or WriteHeader was already called")
+
 // http2Server implements the ServerTransport interface with HTTP2.
 type http2Server struct {
 	conn        net.Conn
@@ -383,7 +387,7 @@ func (t *http2Server) writeHeaders(s *Stream, b *bytes.Buffer, endStream bool) e
 		}
 		if err != nil {
 			t.Close()
-			return ConnectionErrorf("%v", err)
+			return ConnectionErrorf("grpc/transport: %v", err)
 		}
 	}
 	return nil
@@ -394,7 +398,7 @@ func (t *http2Server) WriteHeader(s *Stream, md metadata.MD) error {
 	s.mu.Lock()
 	if s.headerOk || s.state == streamDone {
 		s.mu.Unlock()
-		return fmt.Errorf("transport: the stream is done or WriteHeader was already called")
+		return ErrIllegalHeaderWrite
 	}
 	s.headerOk = true
 	s.mu.Unlock()
@@ -474,7 +478,7 @@ func (t *http2Server) Write(s *Stream, data []byte, opts *Options) error {
 		}
 		if err := t.framer.WriteHeaders(p); err != nil {
 			t.Close()
-			return ConnectionErrorf("%v", err)
+			return ConnectionErrorf("grpc/transport: %v", err)
 		}
 		t.writableChan <- 0
 	}
@@ -522,7 +526,7 @@ func (t *http2Server) Write(s *Stream, data []byte, opts *Options) error {
 		}
 		if err := t.framer.WriteData(s.id, false, p); err != nil {
 			t.Close()
-			return ConnectionErrorf("%v", err)
+			return ConnectionErrorf("grpc/transport: %v", err)
 		}
 		t.writableChan <- 0
 	}
diff --git a/transport/http_util.go b/transport/http_util.go
index 2e2d57996..45c001124 100644
--- a/transport/http_util.go
+++ b/transport/http_util.go
@@ -138,7 +138,7 @@ func newHPACKDecoder() *hpackDecoder {
 		case "grpc-status":
 			code, err := strconv.Atoi(f.Value)
 			if err != nil {
-				d.err = StreamErrorf(codes.Internal, "malformed grpc-status: %v", err)
+				d.err = StreamErrorf(codes.Internal, "grpc/transport: malformed grpc-status: %v", err)
 				return
 			}
 			d.state.statusCode = codes.Code(code)
@@ -149,7 +149,7 @@ func newHPACKDecoder() *hpackDecoder {
 			var err error
 			d.state.timeout, err = timeoutDecode(f.Value)
 			if err != nil {
-				d.err = StreamErrorf(codes.Internal, "malformed time-out: %v", err)
+				d.err = StreamErrorf(codes.Internal, "grpc/transport: malformed time-out: %v", err)
 				return
 			}
 		case ":path":
@@ -175,12 +175,12 @@ func (d *hpackDecoder) decodeClientHTTP2Headers(s *Stream, frame headerFrame) (e
 	d.err = nil
 	_, err = d.h.Write(frame.HeaderBlockFragment())
 	if err != nil {
-		err = StreamErrorf(codes.Internal, "HPACK header decode error: %v", err)
+		err = StreamErrorf(codes.Internal, "grpc/transport: HPACK header decode error: %v", err)
 	}
 
 	if frame.HeadersEnded() {
 		if closeErr := d.h.Close(); closeErr != nil && err == nil {
-			err = StreamErrorf(codes.Internal, "HPACK decoder close error: %v", closeErr)
+			err = StreamErrorf(codes.Internal, "grpc/transport: HPACK decoder close error: %v", closeErr)
 		}
 		endHeaders = true
 	}
@@ -195,12 +195,12 @@ func (d *hpackDecoder) decodeServerHTTP2Headers(s *Stream, frame headerFrame) (e
 	d.err = nil
 	_, err = d.h.Write(frame.HeaderBlockFragment())
 	if err != nil {
-		err = StreamErrorf(codes.Internal, "HPACK header decode error: %v", err)
+		err = StreamErrorf(codes.Internal, "grpc/transport: HPACK header decode error: %v", err)
 	}
 
 	if frame.HeadersEnded() {
 		if closeErr := d.h.Close(); closeErr != nil && err == nil {
-			err = StreamErrorf(codes.Internal, "HPACK decoder close error: %v", closeErr)
+			err = StreamErrorf(codes.Internal, "grpc/transport: HPACK decoder close error: %v", closeErr)
 		}
 		endHeaders = true
 	}
@@ -276,12 +276,12 @@ func timeoutEncode(t time.Duration) string {
 func timeoutDecode(s string) (time.Duration, error) {
 	size := len(s)
 	if size < 2 {
-		return 0, fmt.Errorf("timeout string is too short: %q", s)
+		return 0, fmt.Errorf("grpc/transport: timeout string is too short: %q", s)
 	}
 	unit := timeoutUnit(s[size-1])
 	d, ok := timeoutUnitToDuration(unit)
 	if !ok {
-		return 0, fmt.Errorf("timeout unit is not recognized: %q", s)
+		return 0, fmt.Errorf("grpc/transport: timeout unit is not recognized: %q", s)
 	}
 	t, err := strconv.ParseInt(s[:size-1], 10, 64)
 	if err != nil {
diff --git a/transport/http_util_test.go b/transport/http_util_test.go
index e90e41165..b4b66295c 100644
--- a/transport/http_util_test.go
+++ b/transport/http_util_test.go
@@ -75,9 +75,9 @@ func TestTimeoutDecode(t *testing.T) {
 		err error
 	}{
 		{"1234S", time.Second * 1234, nil},
-		{"1234x", 0, fmt.Errorf("timeout unit is not recognized: %q", "1234x")},
-		{"1", 0, fmt.Errorf("timeout string is too short: %q", "1")},
-		{"", 0, fmt.Errorf("timeout string is too short: %q", "")},
+		{"1234x", 0, fmt.Errorf("grpc/transport: timeout unit is not recognized: %q", "1234x")},
+		{"1", 0, fmt.Errorf("grpc/transport: timeout string is too short: %q", "1")},
+		{"", 0, fmt.Errorf("grpc/transport: timeout string is too short: %q", "")},
 	} {
 		d, err := timeoutDecode(test.s)
 		if d != test.d || fmt.Sprint(err) != fmt.Sprint(test.err) {
diff --git a/transport/transport.go b/transport/transport.go
index cf7e9f1d0..7d7fa813e 100644
--- a/transport/transport.go
+++ b/transport/transport.go
@@ -39,6 +39,7 @@ package transport // import "google.golang.org/grpc/transport"
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -244,13 +245,17 @@ func (s *Stream) StatusDesc() string {
 	return s.statusDesc
 }
 
+// ErrIllegalTrailerSet indicates that the trailer has already been set or it
+// is too late to do so.
+var ErrIllegalTrailerSet = errors.New("grpc/transport: trailer has been set")
+
 // SetTrailer sets the trailer metadata which will be sent with the RPC status
 // by the server. This can only be called at most once. Server side only.
 func (s *Stream) SetTrailer(md metadata.MD) error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if s.trailer != nil {
-		return fmt.Errorf("transport: Trailer has been set")
+		return ErrIllegalTrailerSet
 	}
 	s.trailer = md.Copy()
 	return nil
diff --git a/transport/transport_test.go b/transport/transport_test.go
index 888b5b47b..bac711607 100644
--- a/transport/transport_test.go
+++ b/transport/transport_test.go
@@ -45,9 +45,9 @@ import (
 	"testing"
 	"time"
 
+	"golang.org/x/net/context"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials"
-	"golang.org/x/net/context"
 )
 
 type server struct {