Digital watermarking remains a very empirical area of research so an immediate question which stands out is whether we can develop a comprehensive theory of information hiding, in the sense that Shannon provided us with a proof that one-time pad is secure regardless of the computational power of the opponent. Quite apart from intellectual curiosity, there is a strong practical reason to seek constructions whose security is mathematically provable. This is because copyright protection mechanisms may be subjected to attack over an extraordinarily long period of time. Copyright subsists for typically 50-70 years after the death of the artist, depending on the country and the medium; this means that mechanisms fielded today might be attacked using the resources available in a hundred years' time. This paper gives a general introduction to digital watermarking, its weaknesses and the challenging opportunities for future research. (7 pages)