Watermarks. The term evokes visions of shady characters secretly beavering away in dark basements surrounded by forged $100 bills drying on clothes lines. In a digital media context, away from the traditional world of inks and paper, the same old problem remains but it relates not just to forgery but also to outright theft, because one digital copy can spawn millions of others with the single click of a mouse button. It is hardly surprising that the notion of a digital watermark has stimulated avid interest amongst artists and publishers alike. It is commonly recognized that digital watermarks must be as robust as the media in which they are embedded. For example, a rotated, cropped, and rescanned water-marked image should still be a watermarked image. However, this robustness requirement directly conflicts with the need for a digital watermark to be unobtrusive. The most effective techniques used to embed watermarks are the result of a combination of secret key-based techniques used for military communication and simple models of the human visual system. The most familiar application for digital watermarks is for copyright protection and protection of intellectual property. On its own, a watermark does not provide any legal proof of ownership. In other words, the use of a given digital watermark to protect intellectual property must be registered with a trusted third party to be of any value. Any technique for embedding robust digital watermarks must be compatible with methods for registering copyright. A watermark's resistance to intentional and unintentional degradation has been the main subject of interest in the watermarking community. The main challenges are geometric transformations such as change of proportion or simple rescaling. One watermark removal technique that is supplied on the Internet simply shifts a corner of the image. Lossy image compression such as JPEG and filtering are more easily overcome and, generally speaking, watermarks have evolved into very resistant forms. The laboratory, but will they really work in the real world?