Undergarments, also known as underwear or underclothing, are items of clothing worn beneath outer clothes, usually in direct contact with the skin. They serve to protect outer clothing from sweat and oils, provide support, and offer modesty. Undergarments are generally of two types, those that are worn to cover the torso and those that are worn to cover the waist and legs, although there are also underclothes which cover both. Different styles of underwear are generally worn by females and males. Undergarments commonly worn by females today include bras and panties (knickers in British English), while males often wear classic briefs, boxer briefs, or boxer shorts.

Undergarments are known by a number of terms. Underclothes, underclothing, and underwear are formal terms, while undergarments may be more casually called, in Australia, Reg Grundys (rhyming slang for undies) and Reginalds, and, in the United Kingdom, smalls (from the earlier smallclothes) and (historically) unmentionables. Womens undergarments collectively are also called lingerie. They also are called intimate clothing and intimates. An undershirt (vest in the United Kingdom) is a piece of underwear covering the torso, while underpants (often called pants in the United Kingdom), drawers, and undershorts cover the genitals and buttocks.

Historically, until the early 20th century, shirts were regarded as undergarments, serving the purposes of protection and modesty, and as indicators of wealth or social status. The 1840s saw the introduction of the woollen vest or under-vest, which was "generally made of fine calico". Knitted or flannel undershirts were worn by labouring men because, as well as keeping them warm in winter, they absorbed sweat and were easily washable. Until the 1930s, mens underwear was generally concealing of the body beneath.