The state of marine disease studies is described. Perhaps the greatest area of success in the last 20 years has been in the identification and characterization of viruses, bacteria, fungi, protozoan and metazoan disease agents. The opening of new areas of investigation such as that of interactions between pollutants and infectious agents or non-infectious syndromes such as neoplasia have provided challenges to younger, better equipped investigators in recent efforts. These successes, though not complete in themselves, provide an impetus to understanding complex disease issues. Long standing enigmas, such as complete understanding of the complex life-cycles of devastating pathogens such as protozoans of shellfish, and roles of certain toxicants in fish diseases remain to be better understood.