Many mustelids have a long tube-shaped body, short legs, and a strong, thick neck with a small head. All possess well-developed anal scent glands. The five digits on each foot are equipped with sharp nonretractile claws. Males are usually larger than females; among some weasels males are almost twice as large. A tubular body does not retain heat as well as a stockier body of the same weight and is therefore associated with higher metabolism. As a result, mustelids are very active and inquisitive in their constant search for prey.

Mustelids are mostly solitary except for Eurasian badgers (Meles meles), sea otters (Enhydra lutris), and some northern river otters (Lontra canadensis). In solitary species, association between males and females during the mating season is brief. Mating occurs mostly in the spring, and in many species ovulation is induced during copulation. Delayed implantation of the fertilized egg occurs in many mustelids. Females raise the young alone. Only the least weasel produces two litters yearly; other species produce annually. In most mustelids, young become sexually mature at about 10 months of age. Mustelids evolved from North American and Eurasian forms in the early Oligocene Epoch, some 30 million years ago.