Publisher Summary This chapter briefly introduces two of the major services—Multimedia Telephony (MMTel) and Presence—built on multimedia subsystem (IMS). When devices happen to have incompatible codecs installed, the network can provide conversion services. If the device is a PC and the users are reasonably technically inclined, these issues are normally handled by downloading codecs. However, apart from the inconvenience factor, it is most likely not even possible for the majority of mobile phones. Northbound Interface is an explicit call-out to an external application, invoked as part of the call processing in the MMTel AS node itself. The Presence function in IMS is the result of cooperation between standards bodies, in this case Open Mobile Alliance (OMA), 3GPP and 3GPP2, and the Internet Engineering Task Force (IETF), in a pattern that is becoming common. Basic protocols are developed in IETF; core capabilities and the architecture supporting them is the job of 3GPP, and then organizations like OMA build end-user applications.