A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.