The possibility of adding several watermarks to the same image would enable many interesting applications such as multimedia document tracing, data usage monitoring, and multiple property management. In this book, a novel watermarking scheme is presented, which allows inserting and reliably detecting multiple watermarks sequentially embedded into a digital image. The proposed method, based on elementary linear algebra, is asymmetric, secure under projection attack and robust against distortion due to basic operations such as storage, transmission, and format conversion. Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.