Morbilliviruses infections cause significant mortality in human beings and animals. Measles virus is responsible for up to two million childhood deaths annually in the developing world, while rinderpest and peste des petits ruminants cause severe epizootics in domestic and wild ruminants in areas of the world where they remain endemic. Canine distemper virus (CDV) is a cause of fatal disease in many species of carnivores. Distemper is controlled by vaccination in domestic dogs and farmed mink, but it may be impossible to eradicate the virus because of its global distribution and wide variety of susceptible host species, which includes both freshwater and marine seals. Research is currently under way to develop new recombinant vaccines, since the currently available live attenuated vaccines for CDV are not safe for use for all species and many valuable zoo animals need to be protected from CDV. New morbilliviruses with potentially disastrous ecological consequences for marine mammals have been discovered in the past decade; phocid distemper virus (PDV) in seals and the cetacean morbillivirus (CMV) has been found in dolphins, whales and porpoises. Reverse transcription, coupled with the polymerase chain reaction (RT/PCR) and nucleic acid sequencing, has been used to characterise the morbilliviruses and has given insights into the evolution of this virus genus.