Over the past decade extraordinary advances have been made in the study of protozoan parasites. Particular progress has occurred in areas such as cultivation of protozoan parasites, immunobiology of protozoan parasitic diseases, the biochemistry of protozoa and molecular genetics of these organisms. The application of sophisticated culture, monoclonal antibody and recombinant DNA technologies has resulted in elucidation of many of the biochemical and molecular bases of such phenomena as antigenic variation in African trypanosomas, the autoimmune basis of the pathogenesis of Chagas' disease, protective immunity in malaria and parasite evasion of host defense mechanisms. As a result a new generation of diagnostic procedures have to provide more accurate detection of protozoan infections and thus improved epidemiological information. Vigorous vaccine development efforts are underway which will lead to molecularly defined vaccines taylored to specific applications and will provide new weapons to combat protozoan diseases. Perhaps most importantly the molecular bases of host-parasite interactions are being established and will allow identification of unique biochemical aspects of the biology of protozoa thereby revealing appropriate targets for development of vaccines, accurate detection procedures and more efficacious chemotherapeutic agents.