Current Internet applications, such as e-mail and Web-browsing, are client/server based and do not require session set-up or control. They work very effectively on best-effort IP networks. In the future, it is envisaged that both fixed and mobile services will be provided by a QoS-enabled all-IP core network. The next generation of peer-to-peer style applications, however, require advanced functionality in terms of: user location by name; media negotiation; and session renegotiation on handover, and the like. This functionality is analogous to call control and intelligent network-based call treatment in the PSTN world, except that it is many times richer and potentially more complex. SIP is an excellent candidate for any peer-to-peer applications capable of communication over IP because it is lightweight, flexible, scalable, extensible and programmable. This chapter has shown how the concept of a SIP proxy server can be used to provide personal mobility and allow complicated 'intelligent' services to be delivered. This has included familiar PSTN services, such as call forwarding, as well as new mobile multimedia services. SIP offers a great opportunity for both network and service providers, such as ISPs, to create carefully tailored services, control the process of session initiation to enable higher quality and more reliable services, and enable the use of chargeable network facilities, such as archivers, media codecs and the like.