diff --git a/flow_controller.go b/flow_controller.go
index 2e703ddc..bbc18daf 100644
--- a/flow_controller.go
+++ b/flow_controller.go
@@ -1,6 +1,8 @@
 package quic
 
 import (
+	"sync"
+
 	"github.com/lucas-clemente/quic-go/handshake"
 	"github.com/lucas-clemente/quic-go/protocol"
 )
@@ -16,6 +18,8 @@ type flowController struct {
 	receiveWindowUpdateThreshold      protocol.ByteCount
 	receiveFlowControlWindow          protocol.ByteCount
 	receiveFlowControlWindowIncrement protocol.ByteCount
+
+	mutex sync.RWMutex
 }
 
 func newFlowController(connectionParametersManager *handshake.ConnectionParametersManager) *flowController {
@@ -28,12 +32,18 @@ func newFlowController(connectionParametersManager *handshake.ConnectionParamete
 }
 
 func (c *flowController) AddBytesSent(n protocol.ByteCount) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	c.bytesSent += n
 }
 
 // UpdateSendWindow should be called after receiving a WindowUpdateFrame
 // it returns true if the window was actually updated
 func (c *flowController) UpdateSendWindow(newOffset protocol.ByteCount) bool {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if newOffset > c.sendFlowControlWindow {
 		c.sendFlowControlWindow = newOffset
 		return true
@@ -42,6 +52,9 @@ func (c *flowController) UpdateSendWindow(newOffset protocol.ByteCount) bool {
 }
 
 func (c *flowController) SendWindowSize() protocol.ByteCount {
+	c.mutex.RLock()
+	defer c.mutex.RUnlock()
+
 	if c.bytesSent > c.sendFlowControlWindow { // should never happen, but make sure we don't do an underflow here
 		return 0
 	}
@@ -49,6 +62,9 @@ func (c *flowController) SendWindowSize() protocol.ByteCount {
 }
 
 func (c *flowController) AddBytesRead(n protocol.ByteCount) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	c.bytesRead += n
 }
 
@@ -59,6 +75,9 @@ func (c *flowController) MaybeTriggerBlocked() bool {
 		return false
 	}
 
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if c.lastBlockedSentForOffset == c.sendFlowControlWindow {
 		return false
 	}
@@ -70,6 +89,9 @@ func (c *flowController) MaybeTriggerBlocked() bool {
 // MaybeTriggerWindowUpdate determines if it is necessary to send a WindowUpdate
 // if so, it returns true and the offset of the window
 func (c *flowController) MaybeTriggerWindowUpdate() (bool, protocol.ByteCount) {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	diff := c.receiveFlowControlWindow - c.bytesRead
 	if diff < c.receiveWindowUpdateThreshold {
 		c.receiveFlowControlWindow += c.receiveFlowControlWindowIncrement
@@ -79,6 +101,9 @@ func (c *flowController) MaybeTriggerWindowUpdate() (bool, protocol.ByteCount) {
 }
 
 func (c *flowController) CheckFlowControlViolation(highestByte protocol.ByteCount) bool {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
 	if highestByte > c.receiveFlowControlWindow {
 		return true
 	}
diff --git a/session.go b/session.go
index 56506015..20a15be8 100644
--- a/session.go
+++ b/session.go
@@ -290,20 +290,18 @@ func (s *Session) handleWindowUpdateFrame(frame *frames.WindowUpdateFrame) error
 		// return errors.New("Connection level flow control not yet implemented")
 		return nil
 	}
+
 	s.streamsMutex.RLock()
-	defer s.streamsMutex.RUnlock()
-
 	stream, streamExists := s.streams[frame.StreamID]
-
 	if !streamExists {
 		return errWindowUpdateOnInvalidStream
 	}
 	if stream == nil {
 		return errWindowUpdateOnClosedStream
 	}
+	s.streamsMutex.RUnlock()
 
 	stream.UpdateSendFlowControlWindow(frame.ByteOffset)
-
 	return nil
 }
 
diff --git a/stream.go b/stream.go
index 5459251c..c2719349 100644
--- a/stream.go
+++ b/stream.go
@@ -140,9 +140,6 @@ func (s *stream) ReadByte() (byte, error) {
 }
 
 func (s *stream) UpdateSendFlowControlWindow(n protocol.ByteCount) {
-	s.mutex.Lock()
-	defer s.mutex.Unlock()
-
 	if s.flowController.UpdateSendWindow(n) {
 		s.windowUpdateOrErrCond.Broadcast()
 	}