diff --git a/src/github.com/matrix-org/dendrite/clientapi/auth/auth.go b/src/github.com/matrix-org/dendrite/clientapi/auth/auth.go
new file mode 100644
index 000000000..e3d8e6fae
--- /dev/null
+++ b/src/github.com/matrix-org/dendrite/clientapi/auth/auth.go
@@ -0,0 +1,55 @@
+package auth
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/util"
+)
+
+// VerifyAccessToken verifies that an access token was supplied in the given HTTP request
+// and returns the user ID it corresponds to. Returns resErr (an error response which can be
+// sent to the client) if the token is invalid or there was a problem querying the database.
+func VerifyAccessToken(req *http.Request) (userID string, resErr *util.JSONResponse) {
+	token, tokenErr := extractAccessToken(req)
+	if tokenErr != nil {
+		resErr = &util.JSONResponse{
+			Code: 401,
+			JSON: jsonerror.MissingToken(tokenErr.Error()),
+		}
+		return
+	}
+	if token == "fail" {
+		res := util.ErrorResponse(fmt.Errorf("Fatal error"))
+		resErr = &res
+	}
+	// TODO: Check the token against the database
+	return
+}
+
+// extractAccessToken from a request, or return an error detailing what went wrong. The
+// error message MUST be human-readable and comprehensible to the client.
+func extractAccessToken(req *http.Request) (string, error) {
+	// cf https://github.com/matrix-org/synapse/blob/v0.19.2/synapse/api/auth.py#L631
+	authBearer := req.Header.Get("Authorization")
+	queryToken := req.URL.Query().Get("access_token")
+	if authBearer != "" && queryToken != "" {
+		return "", fmt.Errorf("mixing Authorization headers and access_token query parameters")
+	}
+
+	if queryToken != "" {
+		return queryToken, nil
+	}
+
+	if authBearer != "" {
+		parts := strings.SplitN(authBearer, " ", 2)
+		if len(parts) != 2 || parts[0] != "Bearer" {
+			return "", fmt.Errorf("invalid Authorization header")
+		}
+		return parts[1], nil
+	}
+
+	return "", fmt.Errorf("missing access token")
+}
diff --git a/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go b/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
index a0111197c..ea64896db 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/jsonerror/jsonerror.go
@@ -1,6 +1,9 @@
 package jsonerror
 
-import "fmt"
+import (
+	"fmt"
+	"github.com/matrix-org/util"
+)
 
 // MatrixError represents the "standard error response" in Matrix.
 // http://matrix.org/docs/spec/client_server/r0.2.0.html#api-standards
@@ -13,6 +16,20 @@ func (e *MatrixError) Error() string {
 	return fmt.Sprintf("%s: %s", e.ErrCode, e.Err)
 }
 
+// InternalServerError returns a 500 Internal Server Error in a matrix-compliant
+// format.
+func InternalServerError() util.JSONResponse {
+	return util.JSONResponse{
+		Code: 500,
+		JSON: Unknown("Internal Server Error"),
+	}
+}
+
+// Unknown is an unexpected error
+func Unknown(msg string) *MatrixError {
+	return &MatrixError{"M_UNKNOWN", msg}
+}
+
 // Forbidden is an error when the client tries to access a resource
 // they are not allowed to access.
 func Forbidden(msg string) *MatrixError {
diff --git a/src/github.com/matrix-org/dendrite/clientapi/readers/sync.go b/src/github.com/matrix-org/dendrite/clientapi/readers/sync.go
index 5f4516fb1..c4b3a2474 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/readers/sync.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/readers/sync.go
@@ -3,12 +3,18 @@ package readers
 import (
 	"net/http"
 
+	"github.com/matrix-org/dendrite/clientapi/auth"
 	"github.com/matrix-org/util"
 )
 
 // Sync implements /sync
 func Sync(req *http.Request) util.JSONResponse {
 	logger := util.GetLogger(req.Context())
-	logger.Info("Doing stuff...")
+	userID, resErr := auth.VerifyAccessToken(req)
+	if resErr != nil {
+		return *resErr
+	}
+
+	logger.WithField("userID", userID).Info("Doing stuff...")
 	return util.MessageResponse(404, "Not implemented yet")
 }
diff --git a/src/github.com/matrix-org/dendrite/clientapi/writers/sendmessage.go b/src/github.com/matrix-org/dendrite/clientapi/writers/sendmessage.go
index ae4103da4..bbb432ec8 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/writers/sendmessage.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/writers/sendmessage.go
@@ -3,12 +3,22 @@ package writers
 import (
 	"net/http"
 
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/dendrite/clientapi/auth"
 	"github.com/matrix-org/util"
 )
 
 // SendMessage implements /rooms/{roomID}/send/{eventType}
 func SendMessage(req *http.Request, roomID, eventType string) util.JSONResponse {
 	logger := util.GetLogger(req.Context())
-	logger.WithField("roomID", roomID).WithField("eventType", eventType).Info("Doing stuff...")
+	userID, resErr := auth.VerifyAccessToken(req)
+	if resErr != nil {
+		return *resErr
+	}
+	logger.WithFields(log.Fields{
+		"roomID":    roomID,
+		"eventType": eventType,
+		"userID":    userID,
+	}).Info("Doing stuff...")
 	return util.MessageResponse(404, "Not implemented yet")
 }