Rodents are hosts of many ixodid ticks, which are themselves the natural hosts of rickettsiae. This chapter aims to review briefly the evolution of the major rodent taxa and their distribution as important factors in rickettsial evolution and distribution. The earliest known rodents, all belonging to the extinct family Paramyidae, appear only in late Paleocene deposits of North America and early Eocene deposits in Europe. But this in no way excludes the vast tropical areas of Asia and Africa as the probable birthplace of the Rodentia. Simpson divided the Order Rodentia into three suborders: the Sciuromorpha, most of which probably arose in the Nearctic Region, the Myomorpha, predominantly of Palearctic origin, and the Hystricomorpha, clearly Neotropical in origin and development. The rodent fauna of Australia is composed solely of the Muroidea, more specifically the family Muridae. It is abundantly clear that, at least in terms of rodent fauna, the Oriental Faunal Region cannot be treated as a single homogeneous system.