diff --git a/client.go b/client.go
index 294723a..727f68a 100644
--- a/client.go
+++ b/client.go
@@ -16,6 +16,7 @@ import (
 
 type Client interface {
 	Name() string
+	Password() string
 	Register(bool)
 	Send(*irc.Message)
 	Receive(*irc.Message)
@@ -74,6 +75,10 @@ func (cl *RemoteClient) Name() string {
 	return cl.name
 }
 
+func (cl *RemoteClient) Password() string {
+	return cl.password
+}
+
 func (cl *RemoteClient) Register(success bool) {
 	cl.registered <- success
 }
diff --git a/server.go b/server.go
index 30a3846..c93c483 100644
--- a/server.go
+++ b/server.go
@@ -48,6 +48,8 @@ type Server struct {
 	connectionsCurrent float64
 	connectionsCount   float64
 	queueLen           float64
+
+	authCallback func(name, pass string) bool
 }
 
 // Create a new server instance.
@@ -82,6 +84,10 @@ func NewServer(configPath, software, version string) *Server {
 	return sv
 }
 
+func (sv *Server) SetAuthCallback(authCB func(name, pass string) bool) {
+	sv.authCallback = authCB
+}
+
 // Open the listening port and start the main server loop.
 func (sv *Server) Run() {
 	xlog.Info("%s/%s", sv.software, sv.version)
@@ -141,17 +147,31 @@ func (sv *Server) dispatcher() {
 		case cl := <-sv.addq:
 			clid := strings.ToLower(cl.Name())
 			if _, exists := sv.clients[clid]; exists {
-				cl.Register(false)
-				xlog.Info("Client registration failed: '%s'", clid)
-			} else {
-				sv.clients[clid] = cl
-				sv.sendLogon(cl.Name())
-				sv.connectionsCurrent = float64(len(sv.clients))
-				cl.Register(true)
-				xlog.Info("Client registered: '%s'", clid)
-				xlog.Info("Server has %d client(s)", len(sv.clients))
-				xlog.Debug("Goroutines running: %d", runtime.NumGoroutine())
+				sv.sendReply(cl.Name(), ERR_NICKNAMEINUSE, "", "Nickname is already in use")
+				go func() {
+					time.Sleep(5 * time.Second)
+					cl.Register(false)
+				}()
+				xlog.Info("Client registration failed: '%s' (client exists)", clid)
+				continue
 			}
+			if !sv.authCallback(cl.Name(), cl.Password()) {
+				sv.sendReply(cl.Name(), ERR_PASSWDMISMATCH, "", "Password incorrect")
+				go func() {
+					time.Sleep(5 * time.Second)
+					cl.Register(false)
+				}()
+				xlog.Info("Client registration failed: '%s' (wrong password)", clid)
+				continue
+			}
+			sv.clients[clid] = cl
+			sv.clients[clid] = cl
+			sv.sendLogon(cl.Name())
+			sv.connectionsCurrent = float64(len(sv.clients))
+			cl.Register(true)
+			xlog.Info("Client registered: '%s'", clid)
+			xlog.Info("Server has %d client(s)", len(sv.clients))
+			xlog.Debug("Goroutines running: %d", runtime.NumGoroutine())
 		case cl := <-sv.delq:
 			clid := strings.ToLower(cl.Name())
 			cl.Destroy()
@@ -397,7 +417,6 @@ func handleCmdTopic(sv *Server, msg *irc.Message) {
 		//sv.sendReply(msg.Pre, RPL_TOPIC, ch, msg.Trail)
 	}
 }
-
 func handleCmdNames(sv *Server, msg *irc.Message) {
 	ch := msg.Args[0]
 	if _, exists := sv.chUsers[ch]; !exists {