diff --git a/session.go b/session.go
index a1eb3a707..349050372 100644
--- a/session.go
+++ b/session.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net"
+	"sync"
 
 	"github.com/lucas-clemente/quic-go/frames"
 	"github.com/lucas-clemente/quic-go/handshake"
@@ -27,14 +28,16 @@ type Session struct {
 	ServerConfig *handshake.ServerConfig
 	cryptoSetup  *handshake.CryptoSetup
 
-	EntropyReceived EntropyAccumulator
-	EntropySent     EntropyAccumulator
-	EntropyHistory  map[protocol.PacketNumber]EntropyAccumulator // ToDo: store this with the packet itself
+	EntropyReceived     EntropyAccumulator
+	EntropySent         EntropyAccumulator
+	EntropyHistory      map[protocol.PacketNumber]EntropyAccumulator // ToDo: store this with the packet itself
+	entropyHistoryMutex sync.Mutex
 
 	lastSentPacketNumber     protocol.PacketNumber
 	lastObservedPacketNumber protocol.PacketNumber
 
-	Streams map[protocol.StreamID]*Stream
+	Streams      map[protocol.StreamID]*Stream
+	streamsMutex sync.RWMutex
 
 	streamCallback StreamCallback
 }
@@ -147,7 +150,10 @@ func (s *Session) handleStreamFrame(r *bytes.Reader) error {
 		return errors.New("Session: 0 is not a valid Stream ID")
 	}
 
+	s.streamsMutex.RLock()
 	stream, newStream := s.Streams[frame.StreamID]
+	s.streamsMutex.RUnlock()
+
 	if !newStream {
 		stream, _ = s.NewStream(frame.StreamID)
 	}
@@ -168,6 +174,8 @@ func (s *Session) handleAckFrame(r *bytes.Reader) error {
 		return err
 	}
 
+	s.entropyHistoryMutex.Lock()
+	defer s.entropyHistoryMutex.Unlock()
 	expectedEntropy, ok := s.EntropyHistory[frame.LargestObserved]
 	if !ok {
 		return errors.New("No entropy value saved for received ACK packet")
@@ -257,6 +265,8 @@ func (s *Session) SendFrame(frame frames.Frame) error {
 		return err
 	}
 	s.EntropySent.Add(packetNumber, entropyBit)
+	s.entropyHistoryMutex.Lock()
+	defer s.entropyHistoryMutex.Unlock()
 	s.EntropyHistory[packetNumber] = s.EntropySent
 
 	ciphertext := s.cryptoSetup.Seal(s.lastSentPacketNumber, fullReply.Bytes(), framesData.Bytes())
@@ -269,6 +279,8 @@ func (s *Session) SendFrame(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)