Swimsuits, swimming costumes or bathing suits are clothing designed to be worn while swimming. In ANZAC English, swimsuits are usually called 'togs'. Swimsuits can be skintight or loose fitting and range from modest garments to risqu garments. They are often lined with a fabric that prevents them from becoming transparent when wet; however, there are swimsuits, typically called sheer, which are specifically designed to be transparent when wet. Swimsuits are designed to cover at the genitals and breasts. Men's swimsuit styles are swimming trunks such as shorts, jammers, speedo-style, thongs or g-strings. Women's swimsuits are generally either one-pieces, swimsuits or g-strings/thongs. The monokini, a style of swimsuit that most often takes the form of a swimsuits bottom without a top.



Monokinis are quite common in South America and Europe, though due to taboos they are almost never seen in the United States, except in places with a strong European tourist influence. Special swimsuits for Olympic swimming, designed to reduce drag, can resemble unitards. For some swimming and diving, special bodysuits are worn. These suits are made from spandex and protect the skin from stings and abrasion. Most professional swimmers also wear special swimsuits including bodysuits, racerback, jammers and racing briefs to assist their glide through water and gain speed advantages.

Swimsuits are also worn during pageants. Magazines like SI's annual "swimsuit issue" feature models and sport personalities adorned swimsuits. One-piece swimsuits, or a tank suit, leotard or more simply a one-piece: Probably the most common form of one-piece swimsuit, the tank suit form is inspiration for the subsequent creation of the tank top as a mainstream article of clothing. The name "tank suit" is derived from the term "swimming tank", an obsolete term for what is now called a pool. Monokini: a term used for different styles of one-piece swimsuits inspired by the swimsuits. Most commonly, a monokini is a swimsuits bottom without the corresponding top, worn by women. Sling swimsuits are sometimes, though not often, referred to as monokinis. Thong swimsuit: One-piece swimsuit with thong back, buttocks exposed, otherwise an ordinary swimsuit Sling swimsuits: also known as a "suspender swimsuits", "suspender thong", "slingshot swimsuits" or just "slingshot". The slingshot is a one-piece suit providing little, or even less, coverage as a regular swimsuits. A slingshot resembles a swimsuits, but rather than the straps going around the hips or waist, the side straps extend upwards to cover the breasts and go over the shoulders, leaving the entire sides of the torso uncovered, but the nipples and pubic area covered. Behind the neck, the straps join and reach down the back to become a thong. Pretzel suit: a one-piece suit similar to a sling swimsuits, but the straps encircle the torso around the bottom of the ribcage, forming a very high sided swimsuits bottom; instead of the straps passing over the neck and down the back, they simply encircle the neck, joining the straps which pass around the midriff. Engineers are greatly involved in the development of competitive swimsuits. It is as of late that in effort to improve the effectiveness of the swimsuits, engineers have taken to designing them to replicate the skin of sea based animals, sharks in particular. These swimsuits are created in order to minimize water resistance as much as possible allowing a swimmer to move more efficiently in water.

In Classical antiquity swimming and bathing were done nude. In some settings coverings were used. Murals at Pompeii show women wearing two-piece swimsuits covering the areas around their breasts and hips in a fashion remarkably similar to the swimsuits of 1969. After this, the notion of special water apparel seems to have been lost for centuries. In the 18th century swimsuits for women were termed "bathing gowns" which were long dresses of fabrics, with weights sewn into the hems so that they would not rise up in the water. The men's swimsuit, a rather form-fitting wool garment with long sleeves and legs similar to long underwear, was developed and would change little for a century. In the 19th century, the womens two piece swimsuits became common-the two pieces being a gown from shoulder to knees plus a set of trousers with leggings going down to the ankles. In the Victorian era, popular beach resorts were commonly equipped with bathing machines designed to avoid the exposure of people in swimsuits, especially to people of the opposite sex. In the early 1900s an Australian swimmer visited the United States as an "underwater ballerina", a version of synchronized swimming involving diving into glass tanks. She was arrested for indecent exposure because her swimsuits showed arms, legs and the neck. Kellerman changed the suit to have long arms and legs and a collar, still keeping the close fit that revealed the shapes underneath. She later starred in several movies, including one about her life. After this event, swimsuits shrank, first uncovering the arms and then the legs up to mid-thigh. Collars receded from around the neck down to around the top of the bosom. The development of new fabrics allowed for new varieties of more comfortable and practical swim wear.

Due to the figure-hugging nature of these garments, glamour photography since the 1940s and 1950s has often featured people wearing swimsuits. This subset of glamour photography eventually evolved into swimsuit photography exemplified by the SI annual swimsuit issues. The first swimsuits were introduced just after World War II. Early examples were not very different from the women's two pieces common since the 1920s, except that they had a gap below the breast line allowing for a section of bare midriff. They were named after Bikini Atoll, the site of several nuclear weapons tests, for their supposed explosive effect on the viewer. Through the 1950s, it was proper for the lower part of the swimsuits to come up high enough to cover the belly-button. From the 1960s on, the swimsuits shrank in all directions until it sometimes covered little more than the barest essentials, although less revealing swimsuits giving more support to the breasts remained popular.