In order to understand and analyse the main components of watermarking security presented in the next chapters, we introduce in this chapter the different elements needed to embed a watermark or a message inside a host content. We first present a functional view of a watermarking scheme (the embedding function, the decoding/detection function) and then its geometrical interpretation. Then we present the most popular class of watermarking schemes: spread-spectrum watermarking and watermarking techniques based on the idea of dirty paper codes.