The many and various species of Leishmania are responsible for a broad spectrum of human and animal diseases known collectively as the leishmaniases. They are widely distributed in the warmer parts of the world and transmitted by the bite of infected female phlebotomine sandflies. The life cycle of Leishmania is relatively straightforward; in the mammalian host the organisms are intracellular in the form of amastigotes, and are obligate parasites of cells of the mononuclear phagocyte system. Female sandflies become infected when they take bloodmeals from infected mammals, and ingested amastigotes transform into uniflagellate promastigote forms. The promastigotes are extracellular and found in areas of the fore- and mid-gut of the insect’s alimentary tract. Promastigotes exist in a variety of shapes and sizes in the gut lumen, some are attached to the gut wall by their flagella, and others are free-swimming. The metacyclic forms are small-bodied promastigotes with long flagella, which when injected into a mammal by the sandfly, are responsible for the transfer of infection.