There are approaching 1000 species of sand-flies in six genera, within the subfamily Phlebotominae of the family Psychodidae. Species in three genera – Phlebotomus, Lutzomyia and Sergentomyia – suck blood from vertebrates; the former two are the more important as they contain disease vectors. The genus Phlebotomus occurs only in the Old World, from southern parts of the northern temperate areas, mainly the Mediterranean region, to central Asia, and in tropical areas, but there are not many species in sub-Saharan Africa or South-east Asia and none in the Pacific area. Most Phlebotomus species inhabit semiarid and savannah areas in preference to forests. Lutzomyia species are found only in the New World, and, by contrast, occur mainly in forested areas of Central and South America.

Sergentomyia species are also confined to the Old World, being found mainly in the Indian subregion, sub-Saharan Africa and Asia. Although a few species bite people they are not vectors.

Adult flies are often called sand-flies because of their colour. However, this can be confusing because in some parts of the world the small biting midges of the family Ceratopogonidae (Chapter 4) and black-flies (Simuliidae, Chapter 6) are called sand-flies. The medically most important species include Phlebotomus papatasi, P. sergenti, P. argentipes, P. ariasi, P. perniciosus and species in the Lutzomyia longipalpis and L. flaviscutellata complexes. In both the Old and New Worlds sand-flies are vectors of leishmaniasis and viruses responsible for sandfly fever, and in the Andes the bacterium Bartonella bacilliformis, causing bartonellosis (Carrión's disease).