This Master Thesis discusses SIP, Session Initiation Protocol, which provides services for user location, determination of user availability and media negotiation for the setup of subsequent multimedia sessions. Also discussed is the author's implementations of a few SIP components, and how they could be used together with Marratech Pro, an application for multimedia conferencing developed by Marratech AB. SIP is then examined in terms of its relation to H.323, another protocol for setup and management of multimedia sessions, and the possibilities for SIP to coexist with this protocol.