In this Chapter background knowledge has been reviewed. Signals processed by spatial audio coding and processing are usually played back over stereo or multi-channel surround loudspeaker systems, headphones, or headphones by means of binaural audio playback. The specific playback system is implicitly or explicitly considered by spatial audio coding and processing. Further, conventional audio signal representation and coding has been reviewed since spatial audio coding aims at enabling lower bitrate representation for stereo or multi-channel signals than the conventional techniques. Matrix surround has been described due to its similarity with spatial audio coding in terms of representing a surround audio signal using a down-mix signal. The next chapter describes background information, related to spatial hearing, which spatial audio coding and processing are based on.