Publisher Summary This chapter introduces the Evolved Packet System (EPS) Architecture by presenting a high-level perspective of the complete system as defined in the System Architecture Evolution (SAE) work item. There are several domains in EPS, each one a grouping of logical nodes that interwork to provide a specific set of functions in the network. The Core Network is divided into multiple domains (Circuit Core, Packet Core, and IMS). These domains interwork with each other over a number of well-defined interfaces. The user management domain provides coordinated subscriber information and supports roaming and mobility between and within the different domains. The Circuit Core domain consists of nodes and functions that provide support for circuit-switched services over GSM and WCDMA. The Packet Core domain consists of nodes and functions that provide support for packet-switched services (primarily IP connectivity) over GSM, WCDMA and High Speed Packet Access (HSPA). Furthermore, the Packet Core domain also provides support for packet-switched services over LTE and non-3GPP access networks, which in general are not related to the Circuit Core (except for some specific features needed for. The packet core domain also provides functions for management and enforcement of service and bearer level policies such as QoS.