The development of shellfish-based industries and the concomitant increase in demand for the introduction and transfer of different shellfish species and stocks has increased the risks of spreading their parasites and diseases around the world. To avoid the accidental introduction of infectious disease agents, information on known parasites and diseases must be readily available. Since the number of recognized infectious agents and facts on known diseases is continuously increasing, it is necessary to update the current state of knowledge. Thus, published and new accounts of viruses, bacteria, protozoa, fungi, metazoa, and infectious diseases of unknown aetiology in commercially exploited shellfish (molluscs, echinoderms, and crustaceans) are summarised. The summaries were devised to be of use to regulatory agencies, diagnosticians, researchers, and students who may require information on this diverse subject. The information is organised according to the host that is normally infected (i.e. oysters, mussels, clams, cockles, scallops, abalone, sea urchins, sea stars, lobsters, shrimp, prawns, crabs, and freshwater crayfish). Each of the 169 summaries includes the common or widely accepted name of the parasite or disease agent, and the scientific name (where known) or taxonomic affiliation. In addition, geographic distribution, host species infected (both naturally and experimentally), impact on host health, diagnostic techniques including illustrations for many of the diseases, known methods of control, and appropriate references are provided.