The 20 or so species of Leishmania which have been recorded as human infections are all either zoonotic, or have recent zoonotic origins. Their distribution is determined by that of their vector, their reservoir host, or both, so is dependent on precise environmental features. This concatenation of limiting factors leads to specific environmental requirements and focal distribution of zoonotic or anthroponotic sources. Human infection is dependent on the ecological relationship between human activity and reservoir systems. Examples are available of the emergence of leishmaniasis from the distant past to the present, and can be postulated for the future. These emergences have been provoked by the adoption of new, secondary reservoir hosts, the adoption of new vector species, transport of infection in humans or domestic animals, invasion by humans of zoonotic foci, and irruption of reservoir hosts beyond their normal range. The leishmaniases therefore present an excellent model for emerging disease in general, and for the generation of the principles governing emergence. The model is, however, limited by gaps in our knowledge, usually quantitative, sometimes qualitative, of the structure of reservoir systems.