Underwear, underclothing, or undergarments are items of clothing worn beneath outer clothes, usually in direct contact with the skin, although they may comprise more than a single layer. They serve to keep outer clothing from being soiled or damaged by bodily excretions, to lessen the friction of outerwear against the skin, to shape the body, and to provide concealment or support for parts of it. In cold weather, long underwear is sometimes worn to provide additional warmth. Special types of undergarments have religious significance. Some items of clothing are designed as undergarments, while others, such as T-shirts and certain types of shorts, are appropriate both as underwear and outerwear. If made of suitable material or textile, some underwear can serve as nightwear or swimwear, and some undergarments are intended for sexual attraction or visual appeal.

Boxer briefs are a hybrid type of men's undergarment which are long in the leg, similar to boxer shorts, but tight-fitting like briefs. They emerged as a style in the 1990s and are commonly worn for sports and as every-day underwear.