There are many different kinds of associations between species of organisms. Parasitism is perhaps the most intimate of these, involving very tight integration between the biologies of the interacting species. Contrary to popular belief, parasitic relationships are highly varied and parasitism can be extraordinarily difficult to define. The associations between organisms are often so subtle as to defy easy description. Nowhere is the subtlety and variety more apparent than in the sea. Most marine animal phyla include parasitic members and most marine animals are themselves susceptible to infection by co-evolved parasitic species. An overview is given of the range of such associations in the sea and the phylogenetic diversity of both parasites and hosts. Other topics covered include: problems faced by parasites and the ways in which they have overcome these; evolutionary and ecological aspects of being a marine parasite; economic and medical marine parasitology.