Among the countless examples of interspecific interactions in biology, surely non-could be more interesting than which established between host animals and their specific parasites. The members of the genus Leishmania are peculiar obligate intracellular protozoans in that they proliferate in the enzyme rich phagolysosomes, an organelle which is essential component of all defence reactions of animals. These parasites are responsible for considerable morbidity and mortality worldwide and have, consequently, been the subject of much study, which has had the underlying objective of discovering the means of controlling these organisms. In this article some immunobiological aspects of leishmaniasis will be reviewed.