Digital watermarking has recently gained an intense interest in research and applications. In this book, the relations and differences between watermarking and communication systems are elaborated. Based on these results new methods for both watermarking and communication are derived. A new blind, robust and reversible watermarking scheme based on Code Division Multiple Access (CDMA) is presented. Using this scheme watermark is arithmetically added to spatial domain or frequency domain. Watermark is extracted by using spreading codes only. Proposed watermarking scheme is simple, computationally efficient and can be applied to any image format. Presented robust watermarking scheme can easily be transformed into a reversible watermarking scheme. A novel idea that watermark can be part of the image is also presented. By using watermark, which is a part of an image, digital watermarking can be used beyond simple security tasks. Furthermore, CDMA based watermarking is used to authenticate radio frequency signal.