In computing, SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions) is an instant messaging (IM) protocol. In contrast to the vast majority of IM software deployed today, SIMPLE is an open standard.

SIMPLE applies the Session Initiation Protocol to the problems of:

• Registering for Presence Information and receiving notifications when such events occur, for example when a user logs-in or comes back from lunch.
• Managing a session of real-time messages between two or more participants.

SIMPLE is currently (2003) IETF work in progress, taking place in the simple workgroup (http://www.ietf.org/html.charters/simple-charter.html). Some parts have been standardized, e.g. RFC 3428 (http://www.ietf.org/rfc/rfc3428.txt). Other parts, in particular IM sessions, are still under discussion. However, several implementations are already available, notably including the Microsoft Windows Messenger.

SIMPLE is not the only IM protocol being developed within the IETF. XMPP, based on the Jabber technology, is a competitor.