Hamsters are rodents (order Rodentia) belonging to the subfamily Cricetinae, which contains 19 species classified in seven genera. They have become established as popular small pets. The best-known species of hamster is the golden or Syrian hamster (Mesocricetus auratus), which is the type most commonly kept as a pet. Other hamster species commonly kept as pets are the three species of dwarf hamster, Campbell's dwarf hamster (Phodopus campbelli), the winter white dwarf hamster (Phodopus sungorus) and the Roborovski hamster (Phodopus roborovskii), and the less common Chinese hamster (Cricetulus griseus).

Hamsters feed primarily on seeds, fruits, vegetation, and occasionally burrowing insects. In the wild, they are crepuscular: they forage during the twilight hours. In captivity, however, they are known to live a conventionally nocturnal lifestyle, waking around sundown to feed and exercise. Physically, they are stout-bodied with distinguishing features that include elongated cheek pouches extending to their shoulders, which they use to carry food back to their burrows, as well as a short tail and fur-covered feet.

Taxonomists generally disagree about the most appropriate placement of the subfamily Cricetinae within the superfamily Muroidea. Some place it in a family Cricetidae that also includes voles, lemmings, and New World rats and mice; others group all these into a large family called Muridae. Their evolutionary history is recorded by 15 extinct fossil genera and extends back 11.2 million to 16.4 million years to the Middle Miocene Epoch in Europe and North Africa; in Asia it extends 6 million to 11 million years. Four of the seven living genera include extinct species. One extinct hamster of Cricetus, for example, lived in North Africa during the Middle Miocene, but the only extant member of that genus is the European or common hamster of Eurasia.

Neumann et al. (2006) conducted a molecular phylogenetic analysis of 12 of the above 17 species using DNA sequence from three genes: 12S rRNA, cytochrome b, and von Willebrand factor. They uncovered the following relationships:

The genus Phodopus was found to represent the earliest split among hamsters. Their analysis included both species. The results of another study suggest Urocricetus kamensis and the related U. alticola belong to either this Phodopus group or hold a similar basal position.

The genus Mesocricetus also forms a clade. Their analysis included all four species, with M. auratus and M. raddei forming one subclade and M. brandti and M. newtoni another.

The remaining genera of hamsters formed a third major clade. Two of the three sampled species within Cricetulus represent the earliest split. This clade contains C. barabensis (and presumably the related C. sokolovi) and C. longicaudatus.

The remaining clade contains members of Allocricetulus, Tscherskia, Cricetus, and C. migratorius. Allocricetulus and Cricetus were sister taxa. Cricetulus migratorius was their next closest relative, and Tscherskia was basal.