This chapter provides a methodological description of the IMS network architecture. IMS service development requires thorough understanding of the principles and protocols of multimedia over IP and of the architecture of the IMS network. Principles and protocols of multimedia over IP have been explained in previous chapters. This chapter builds on that; it focuses on the principles and protocols in applied architecture, and on the IMS network, and explains the reasoning behind the IMS architecture. This involves functional decomposition of the IMS network into the core network (control plane, user plane, Location register), media plane (media transmission), access network (mobile, wireless, wireline), and application layer.