diff --git a/source/ircd/connection.d b/source/ircd/connection.d
index dfb6dc0..6825f31 100644
--- a/source/ircd/connection.d
+++ b/source/ircd/connection.d
@@ -181,6 +181,10 @@ class Connection
 					if(!registered) sendErrNotRegistered();
 					else onLusers(message);
 					break;
+				case "ISON":
+					if(!registered) sendErrNotRegistered();
+					else onIson(message);
+					break;
 				default:
 					writeln("unknown command '", message.command, "'");
 					send(Message(_server.name, "421", [nick, message.command, "Unknown command"]));
@@ -207,11 +211,11 @@ class Connection
 			return;
 		}
 
-                if(!_server.isValidNick(newNick))
-                {
-                        send(Message(_server.name, "432", [nick, newNick, "Erroneous nickname"]));
-                        return;
-                }
+		if(!_server.isValidNick(newNick))
+		{
+			send(Message(_server.name, "432", [nick, newNick, "Erroneous nickname"]));
+			return;
+		}
 
 		if(nick !is null)
 		{
@@ -641,6 +645,22 @@ class Connection
 		_server.sendLusers(this);
 	}
 
+	void onIson(Message message)
+	{
+		if(message.parameters.length < 1)
+		{
+			sendErrNeedMoreParams(message.command);
+			return;
+		}
+
+		//NOTE: The RFCs are ambiguous about the parameter(s).
+		//		It specifies one allowed parameter type, a space-separated list of nicknames (i.e. prefixed with ':').
+		//		However, the nicknames in the example are sent as separate parameters, not as a single string prefixed with ':'.
+		//		For this implementation, we assume the example is wrong, like most clients seem to assume as well.
+		//		(Other server implementations usually seem to support both interpretations.)
+		_server.ison(this, message.parameters[0].split);
+	}
+
 	void sendWhoReply(string channel, Connection user, uint hopCount)
 	{
 		auto flags = user.modes.canFind('a') ? "G" : "H";
diff --git a/source/ircd/server.d b/source/ircd/server.d
index 43617dd..e6f0b02 100644
--- a/source/ircd/server.d
+++ b/source/ircd/server.d
@@ -314,7 +314,7 @@ class Server
 
 	void sendLusers(Connection connection)
 	{
-		//TODO: If RFC-strictness is off, use '1 server' instead of '1 servers' of the network (or the part of the network of the query) has only one server
+		//TODO: If RFC-strictness is off, use '1 server' instead of '1 servers' if the network (or the part of the network of the query) has only one server
 
 		//TODO: Support services and multiple servers
 		connection.send(Message(name, "251", [connection.nick, "There are " ~ connections.filter!(c => c.registered).count.to!string ~ " users and 0 services on 1 servers"], true));
@@ -337,6 +337,13 @@ class Server
 		connection.send(Message(name, "255", [connection.nick, "I have " ~ connections.length.to!string ~ " clients and 1 servers"], true));
 	}
 
+	void ison(Connection connection, string[] nicks)
+	{
+		auto reply = nicks.filter!(n => connections.canFind!(u => u.nick == n)).join(' ');
+
+		connection.send(Message(name, "303", [connection.nick, reply], true));
+	}
+
 	void listen(ushort port = 6667)
 	{
 		listenTCP(port, &acceptConnection);