The Internet evolution started from a small network linking a few research centres to a massive network with billions of computers. The reason behind the growth of the Internet has been its ability to provide very useful services such as World Wide Web, email, instant messaging, VoIP and video conferencing. On the other hand, cellular networks have experienced dramatic growth over the years. The cellular network growth was not only due to its services such as voice and video calls and short messaging services, but also because cellular network users can access the network from virtually everywhere. These facts prompted 3GPP to come up with the idea of the IP Multimedia Subsystem. The IP Multimedia Subsystem aims at merging cellular networks and the Internet, two of the most successful infrastructures in telecommunication. By merging the two infrastructures, the IP Multimedia Subsystem will be able to provide ubiquitous cellular access to all services that are provided by the Internet.