diff --git a/irc/commands.go b/irc/commands.go
index 62676a2d..77224e4b 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -20,6 +20,7 @@ var (
 	parseCommandFuncs  = map[string]parseCommandFunc{
 		"AWAY":    NewAwayCommand,
 		"CAP":     NewCapCommand,
+		"ISON":    NewIsOnCommand,
 		"JOIN":    NewJoinCommand,
 		"MODE":    NewModeCommand,
 		"NICK":    NewNickCommand,
@@ -631,3 +632,22 @@ func NewAwayCommand(args []string) (editableCommand, error) {
 
 	return cmd, nil
 }
+
+type IsOnCommand struct {
+	BaseCommand
+	nicks []string
+}
+
+func (msg *IsOnCommand) String() string {
+	return fmt.Sprintf("ISON(nicks=%s)", msg.nicks)
+}
+
+func NewIsOnCommand(args []string) (editableCommand, error) {
+	if len(args) == 0 {
+		return nil, NotEnoughArgsError
+	}
+
+	return &IsOnCommand{
+		nicks: args,
+	}, nil
+}
diff --git a/irc/reply.go b/irc/reply.go
index f275e373..5b7c5072 100644
--- a/irc/reply.go
+++ b/irc/reply.go
@@ -271,7 +271,14 @@ func RplAway(server *Server, client *Client) Reply {
 		"%s :%s", client.nick, client.awayMessage)
 }
 
+func RplIsOn(server *Server, nicks []string) Reply {
+	return NewNumericReply(server, RPL_ISON,
+		":%s", strings.Join(nicks, " "))
+}
+
+//
 // errors (also numeric)
+//
 
 func ErrAlreadyRegistered(source Identifier) Reply {
 	return NewNumericReply(source, ERR_ALREADYREGISTRED,
diff --git a/irc/server.go b/irc/server.go
index f519cb72..11b96f5d 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -422,3 +422,16 @@ func (msg *AwayCommand) HandleServer(server *Server) {
 		client.Reply(RplUnAway(server))
 	}
 }
+
+func (msg *IsOnCommand) HandleServer(server *Server) {
+	client := msg.Client()
+
+	ison := make([]string, 0)
+	for _, nick := range msg.nicks {
+		if _, ok := server.clients[nick]; ok {
+			ison = append(ison, nick)
+		}
+	}
+
+	client.Reply(RplIsOn(server, ison))
+}