|Today, there are still different technologies used in fixed and mobile networks environment such as cellular technologies (GSM, UMTS, 3G, etc.), wireless network technology like WLAN, wired network technology like ADSL, etc. With the usage of IMS (IP Multimedia Subsystem), all those technologies can be combined together in a fixed mobile convergence environment based on an IP-based infrastructure. Although IMS applies well for a Fixed Mobile convergent environment, there are still issues that need to be solved. First, the IMS central protocol is SIP but there are differences between the 3GPP SIP specifications and the IETF one. This will lead to interoperability problem with different SIP specifications, and the devices between different SIP environments cannot communicate with each other. Second, IMS for a fixed mobile environment should also allow the users to subscribe to any type of services (fixed or mobile), to get access to services on an arbitrary number of registered mobile and fixed devices interchangeably, to dynamically add or remove the number of registered fixed devices. In fact, the general IMS infrastructure is only a high-level scheme and does not support the mentioned requirements. Therefore, in order to satisfy all the mentioned requirements it is crucial to have a sound identity management solution. The goal of this thesis work is to propose a sound identity management solution for a fixed mobile convergent IMS environment. More specifically, the thesis work will be aiming at the following objectives: To provide a concise but clear introduction to Identity Management To provide a comprehensive description on how identities are organized and managed in both fixed and mobile networks To propose an identity management solution for a fixed mobile convergent IMS environment To demonstrate the soundness and feasibility of the proposed solution via the implementation of a prototype. Two Identity Management solutions have been proposed and analyzed as follows: 1. Modified IMS and SSO-enabled SIP system 2. Modified SIP-enabled Client Due to time limitation, only the second solution is selected and a proof-of concept has been successfully designed and implemented. The proof-of-concept has demonstrates the following features: Single-Sign-On between the Mobile and Fixed domain enabling a mobile phone moving from the IMS mobile network to a SIP WLAN network without re-authentication. Identity Federation between the Mobile and Fixed domain enabling the delivery of calls addressed to one domain at the other domain.