This chapter is concerned with stereophonic systems and techniques that use more than two channels or audio streams, often referred to as surround sound, immersive, or 3D audio. It describes formats and principles of surround/immersive audio systems, while the second deals with various forms of practical or operational implementation. The sound object streams will usually be accompanied by metadata that describe how the scene is constructed, what objects there are, where they should be placed, and so forth. This type of representation can be quite relevant to game audio or virtual reality production, where audiovisual objects are created synthetically and there is a lot of source movement or user interaction. Channel-based formats are those where each audio channel in a final mix, broadcast, or data stream is directly fed to a specific loudspeaker channel for reproduction. Object-based audio is a means of describing spatial audio scenes and their content.