A number of diseases affecting lobsters (shell disease, fungal infections and a few selected parasitic occurrences) are described and have been discussed briefly. The bacterial disease, gaffkemia, is described in more detail and used insofar as possible to illustrate the interaction of a pathogen with a vulnerable crustacean host. Emphasis has been placed on the holistic approach stressing the capacity of lobsters and other crustaceans to cope with disease through flexible defense mechanisms, including on occasion the development of resistance.