In recent years, serious disease outbreaks among seals and dolphins were attributed to infection with established or newly recognized morbilliviruses. The first identification of a morbillivirus as causative agent of mass mortality among marine mammals was in 1988, when the previously unrecognized phocine distemper virus (PDV) caused the death of 20,000 harbor seals (Phoca vitulina) in northwestern Europe. A similar epizootic among Baikal seals (Phoca sibirica) in Siberia in 1987 was later attributed to infection with canine distemper virus (CDV). A morbillivirus isolated from stranded harbor porpoises (Phocoena phocoena) between 1988 and 1990 proved to be yet another new member of the genus Morbillivirus, distinct from PDV and CDV and more closely related to rinderpest virus and peste-des-petits-ruminants virus: porpoise morbillivirus. A similar virus, dolphin morbillivirus, was the primary cause of mass mortality among striped dolphins (Stenella coeruleoalba) in the Mediterranean from 1990 to 1992. In this review, current knowledge of the genetic and antigenic relationships of these viruses is presented, and the origin and epizootiological aspects of the newly discovered morbilliviruses are discussed. In addition, the possible contributory role of environmental contaminant-related immunosuppression in the severity and extent of the different disease outbreaks is discussed.