Mustelidae (from Latin mustela, weasel), commonly referred to as the weasel family, are a family of carnivorous mammals. Mustelids are diverse and the largest family in the order Carnivora, at least partly because in the past it has been a catch-all category for many early or poorly differentiated taxa.^{[citation needed]} The internal classification seems to be still quite unsettled, with rival proposals containing between two and eight subfamilies. One study published in 2008 questions the long-accepted Mustelinae subfamily, and suggests Mustelidae consists of four major clades and three much smaller lineages.

The Mustelidae in general are phylogenetically relatively primitive, so were difficult to classify until genetic evidence became available. The increasing availability of such evidence may well result in some members of the family being moved to their own separate families, as has already happened with the skunks, previously considered to be members of the mustelid family.

Mustelids vary greatly in size and behavior. The least weasel is not much larger than a mouse, while the giant otter can measure up to 2.4 metres (7.9 ft) in total length and sea otters can exceed 45 kilograms (99 lb). The wolverine can crush bones as thick as the femur of a moose to get at the marrow, and has been seen attempting to drive bears away from its kill. The sea otter uses rocks to break open shellfish to eat. The marten is largely arboreal, while the badger digs extensive networks of tunnels, called setts. Some mustelids have been domesticated. The ferret and the tayra are kept as pets (although the tayra requires a Dangerous Wild Animals licence in the UK), or as working animals for hunting or vermin control. Others have been important in the fur trade. The mink is often raised for its fur.