In its programme for accelerated development of vaccines for viral respiratory and enteric tract diseases the WHO has assigned a very high priority to respiratory syncytial virus (RSV), parainfluenza viruses and rotaviruses. There is also some interest in alternative approaches to immunization against influenza viruses because of the failure of inactivated vaccines to provide complete and reasonably durable immunity. Current attempts to develop satisfactorily attenuated viruses for use in prevention of disease caused by the above viral pathogens are described.