An instant messaging system and a method of instant messaging between a plurality of clients having IM applications of the same or different types. The IM system includes a plurality of clients having IM client applications of the following types: a PC-based instant messaging client application program (101, 301); a GSM device (104, 303) forming part of a GSM network; an internet browser-based client application 201, 305; and an email-based client application (208, 307). An IM system host (141, 211, 316) including an IM server (143, 217, 311) is selectively connected to each of the clients via the direct electronic links or the internet (117, 213, 309) and provides a prescribed range of functionality to the clients. This functionality includes sending an IM, receiving an IM and identifying which members of a group of clients that a client is a member, or prospective member, of are online. Each client type has a unique identifier to enable access thereto via the internet (117, 213, 309) and each client has a single account on the IM host/server for all of its client types that can access the IM server (143, 217, 311). The account is identified by a unique identification number ("UIN") common to all of the client types of that client and each unique identifier of each client type of a client is matched to the UIN of the particular client.