This chapter is a comprehensive presentation of spread spectrum-based digital audio watermarking methods. The problem is viewed as the realization of a basic communications system, where the host signal presents the available channel and the watermark presents the transmitted information that needs to survive and be recovered in conditions that include noise distortion, signal transformation, standard compression, and deliberate attacks. Basic spread spectrum theory as it relates to audio watermarking is introduced followed by state-of-the-art improvements. The important synchronization problem is analyzed in detail, existing techniques are presented and a novel, precise synchronization method is included. Finally, the role of psychoacoustics in effective watermarking is emphasized and an enhanced psychoacoustic model based on the discrete wavelet packet transform (DWPT), which ensures efficiency and transparency is included.