Parasitic diseases caused by helminths and protozoa are major causes of human disease and misery in most countries of the tropics. They plague billions of people and kill millions annually, and inflict debilitating injuries such as blindness and disfiguration on additional millions. WHO estimates that one person in every four harbors parasitic worms. Attempts to develop vaccines against these pathogens have been hampered by the difficulty to cultivate them in vitro, the complexity of their multicellular organization and/or multistage development, added to their impressive antigenic variability. Although remarkable progress has been made in the last decade in the cloning and expression of protective antigens from a large number of parasites, the prospect of using these antigens for the development of preventive vaccines has been met with little enthusiasm from industrial vaccine manufacturers, due to general scepticism as to the capacity for defined antigens to elicit sterilizing immunity against complex organisms, especially metazoan organisms. Definite scientific and technical advances have nevertheless been made in recent years in the field, including the complete sequencing of the genome of Plasmodium falciparum, and quite a number of groups are now supporting research on vaccine development against parasitic diseases. Significant progress has been made over the past five years in the development of vaccines against malaria and leishmaniasis. Vaccine development efforts for Chagas’ disease (American trypanosomiasis) have been curtailed because of successful efforts at vector control, whereas vaccine development for African sleeping sickness (African trypanosiomiasis) still is hampered by the phenomenon of antigenic variability.