This chapter introduces and describes the two extant species of beavers, the Eurasian Castor fiber and the North American Castor canadensis, including their classification from the order Rodentia, family Castoridae. The Castoridae family represents species that can produce, store, and distribute castoreum. This chapter describes the appearance of these two species in the fossil record and their evolution into two distinct species. Differences between the two species are discussed including chromosome number, appearance, behaviour, and ecology. Background information on extinct beaver species including Castoroides ohioensis—a giant beaver, the size of a bear—is provided. Also considered are the latest genetic findings on subspecies division of the Eurasian beaver which, as for its American counterpart, have been controversial and largely based on fur trade refugia populations. Similar species, which may be confused by some for beavers, are discussed and key differences noted.