This chapter deals with the description of the various applied Mobile System Architectures, showing the evolution path towards the IP Convergence issue, with the introduction of the IP Multimedia Subsystem. It contains the most important networks entities of the different Mobile Networks Systems and their integration to the IMS. Being the core of the Fixed-Mobile Converge, IMS’ operation, functionality and interoperability with the other Telecom platforms are analyzed. Furthermore, aiming in a cost effective high QoS solution, typical performance evaluation strategies of the network manufacturers are described. Its scope is to give an overview of the existing architectures, their network components, their characteristics and their differences, while also show how nearly all traditional Telecom networks can be converged, with the use of the IMS, to an all-IP network, where various applications can be accessed by heterogeneous network platforms.