This chapter describes RFC 4566 that obsoletes RFCs 2327 and 3266. The Session Description Protocol (SDP) is designed to negotiate multimedia sessions that may consist of various features/capabilities/functionalities of audio, video, and/or data application between the communicating entities facilitating the common features/capabilities/functionalities of each application that are agreeable to support by the parties. In fact, SDP may not be a termed as a “protocol” per se, as it only provides the description/format of multimedia session parameters. This section defines the basic characteristics of the SDP describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. In addition, all terminologies and their definitions, and Augmented Backus-Naur Form (ABNF) related to SDP have been put together from all Request for Comments (RFCs) in this chapter.