Members of the genus Morbillivirus classified within the family of Paramyxoviridae are rinderpest virus (RPV), peste des petits ruminants (PPRV), canine distemper virus (CDV), and measles virus (MV) (16). Recently, other morbilliviruses have been isolated from aquatic mammals, like seals (phocid distemper virus, PDV) and dolphins (14,38). All viruses are highly contagious for their natural hosts and may cause serious, acute diseases that may result in high mortality rates. At present vaccination programs with live attenuated vaccines have proven to be the most effective approach to prevent RPV infection in cattle or domestic buffalo populations, PPRV infection in goats and sheep, CDV infection in dogs, and MV infection in humans. The use of live vaccines still has several disadvantages: they are less effective in the presence of maternal antibodies or intercurrent virus infections, may cause severe side-effects in immunocompromized hosts, and need a cold-chain during transport and storage. The use of live attenuated vaccines for use in wild species should not be advocated, as calamities in different wild species with live CDV vaccines have been reported (29). The introduction of Tween-ether- or formaldehyde-inactivated morbillivirus vaccines has been accompanied by serious problems. Animals or humans were shown to be insufficiently protected and in some cases illness following exposure to live virus was more severe than that in nonvaccinated individuals. As a result of new developments in biology and technology, also in the field of morbillivirus vaccines, the development is rapidly changing from the use of conventional methods to novel approaches for vaccine strategies.