diff --git a/session.go b/session.go
index 891c23b83..2902ecbb4 100644
--- a/session.go
+++ b/session.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"sync"
 	"time"
 
 	"github.com/lucas-clemente/quic-go/ackhandler"
@@ -29,7 +30,8 @@ type Session struct {
 	Connection        *net.UDPConn
 	CurrentRemoteAddr *net.UDPAddr
 
-	Streams map[protocol.StreamID]*Stream
+	Streams      map[protocol.StreamID]*Stream
+	streamsMutex sync.RWMutex
 
 	outgoingAckHandler ackhandler.OutgoingPacketAckHandler
 	incomingAckHandler ackhandler.IncomingPacketAckHandler
@@ -131,16 +133,21 @@ func (s *Session) handleStreamFrame(frame *frames.StreamFrame) error {
 	if frame.StreamID == 0 {
 		return errors.New("Session: 0 is not a valid Stream ID")
 	}
-	stream, newStream := s.Streams[frame.StreamID]
+	s.streamsMutex.RLock()
+	stream, existingStream := s.Streams[frame.StreamID]
+	s.streamsMutex.RUnlock()
 
-	if !newStream {
+	if !existingStream {
 		stream, _ = s.NewStream(frame.StreamID)
 	}
+	if stream == nil {
+		return errors.New("Session: reopening streams is not allowed")
+	}
 	err := stream.AddStreamFrame(frame)
 	if err != nil {
 		return err
 	}
-	if !newStream {
+	if !existingStream {
 		s.streamCallback(s, stream)
 	}
 	return nil
@@ -190,6 +197,8 @@ func (s *Session) QueueFrame(frame frames.Frame) error {
 
 // NewStream creates a new strean open for reading and writing
 func (s *Session) NewStream(id protocol.StreamID) (*Stream, error) {
+	s.streamsMutex.Lock()
+	defer s.streamsMutex.Unlock()
 	stream := NewStream(s, id)
 	if s.Streams[id] != nil {
 		return nil, fmt.Errorf("Session: stream with ID %d already exists", id)
@@ -197,3 +206,11 @@ func (s *Session) NewStream(id protocol.StreamID) (*Stream, error) {
 	s.Streams[id] = stream
 	return stream, nil
 }
+
+// closeStream is called by a stream to signal that it was closed remotely
+// and has fininshed reading its data.
+func (s *Session) closeStream(id protocol.StreamID) {
+	s.streamsMutex.Lock()
+	s.Streams[id] = nil
+	s.streamsMutex.Unlock()
+}
diff --git a/session_test.go b/session_test.go
index 5984e7bdb..e4bda6711 100644
--- a/session_test.go
+++ b/session_test.go
@@ -1,6 +1,8 @@
 package quic
 
 import (
+	"io"
+
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 
@@ -56,11 +58,35 @@ var _ = Describe("Session", func() {
 		})
 
 		It("closes streams", func() {
-
+			session.handleStreamFrame(&frames.StreamFrame{
+				StreamID: 5,
+				Data:     []byte{0xde, 0xca, 0xfb, 0xad},
+				FinBit:   true,
+			})
+			Expect(session.Streams).To(HaveLen(1))
+			Expect(session.Streams[5]).ToNot(BeNil())
+			Expect(callbackCalled).To(BeTrue())
+			p := make([]byte, 4)
+			_, err := session.Streams[5].Read(p)
+			Expect(err).To(Equal(io.EOF))
+			Expect(p).To(Equal([]byte{0xde, 0xca, 0xfb, 0xad}))
+			Expect(session.Streams).To(HaveLen(1))
+			Expect(session.Streams[5]).To(BeNil())
 		})
 
 		It("rejects streams that existed previously", func() {
-
+			session.handleStreamFrame(&frames.StreamFrame{
+				StreamID: 5,
+				Data:     []byte{},
+				FinBit:   true,
+			})
+			_, err := session.Streams[5].Read([]byte{0})
+			Expect(err).To(Equal(io.EOF))
+			err = session.handleStreamFrame(&frames.StreamFrame{
+				StreamID: 5,
+				Data:     []byte{},
+			})
+			Expect(err).To(MatchError("Session: reopening streams is not allowed"))
 		})
 	})
 })
diff --git a/stream.go b/stream.go
index 6cff4c766..d80b4a61d 100644
--- a/stream.go
+++ b/stream.go
@@ -9,9 +9,14 @@ import (
 	"github.com/lucas-clemente/quic-go/utils"
 )
 
+type streamHandler interface {
+	QueueFrame(frames.Frame) error
+	closeStream(protocol.StreamID)
+}
+
 // A Stream assembles the data from StreamFrames and provides a super-convenient Read-Interface
 type Stream struct {
-	Session        *Session
+	Session        streamHandler
 	StreamID       protocol.StreamID
 	StreamFrames   chan *frames.StreamFrame
 	CurrentFrame   *frames.StreamFrame
@@ -23,7 +28,7 @@ type Stream struct {
 }
 
 // NewStream creates a new Stream
-func NewStream(session *Session, StreamID protocol.StreamID) *Stream {
+func NewStream(session streamHandler, StreamID protocol.StreamID) *Stream {
 	return &Stream{
 		Session:      session,
 		StreamID:     StreamID,
@@ -56,6 +61,7 @@ func (s *Stream) Read(p []byte) (int, error) {
 				s.currentErr = io.EOF
 				close(s.StreamFrames)
 				s.CurrentFrame = nil
+				s.Session.closeStream(s.StreamID)
 				return bytesRead, io.EOF
 			}
 			s.CurrentFrame = nil
@@ -131,6 +137,7 @@ func (s *Stream) ReadByte() (byte, error) {
 	return p[0], nil
 }
 
+// TODO: Test
 func (s *Stream) Write(p []byte) (int, error) {
 	data := make([]byte, len(p))
 	copy(data, p)
@@ -166,5 +173,6 @@ func (s *Stream) AddStreamFrame(frame *frames.StreamFrame) error {
 // stream should be closed.
 func (s *Stream) RegisterError(err error) {
 	s.currentErr = err
+	s.Session.closeStream(s.StreamID)
 	close(s.StreamFrames)
 }
diff --git a/stream_test.go b/stream_test.go
index 8ee03aa53..5853c4cec 100644
--- a/stream_test.go
+++ b/stream_test.go
@@ -6,17 +6,32 @@ import (
 	"time"
 
 	"github.com/lucas-clemente/quic-go/frames"
+	"github.com/lucas-clemente/quic-go/protocol"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 )
 
+type mockStreamHandler struct {
+	closedStream bool
+}
+
+func (*mockStreamHandler) QueueFrame(frames.Frame) error {
+	panic("not implemented")
+}
+
+func (m *mockStreamHandler) closeStream(protocol.StreamID) {
+	m.closedStream = true
+}
+
 var _ = Describe("Stream", func() {
 	var (
-		stream *Stream
+		stream  *Stream
+		handler *mockStreamHandler
 	)
 
 	BeforeEach(func() {
-		stream = NewStream(nil, 1337)
+		handler = &mockStreamHandler{}
+		stream = NewStream(handler, 1337)
 	})
 
 	It("reads a single StreamFrame", func() {
@@ -274,6 +289,7 @@ var _ = Describe("Stream", func() {
 	Context("closing", func() {
 		AfterEach(func() {
 			Expect(stream.StreamFrames).To(BeClosed())
+			Expect(handler.closedStream).To(BeTrue())
 		})
 
 		Context("with fin bit", func() {