Almost every piece of sports clothing is designed to be light weight so the athlete is not encumbered by its weight. The best athletic wear for some forms of exercise, for example cycling, should not create drag or be too bulky. On the other hand it should be loose enough so as not to restrict movement.
Clothing worn for some other forms of exercise, should not unduly restrict movement and may also have specific requirements for example the Keikogi used in karate. It should allow freedom of movement in competition. Various physically dangerous sports require protective gear, e.g. for fencing, American football or ice hockey.
Standardised sportswear may also have the function of a uniform. In team sports the opposing sides are usually made identifiable by the colours of their clothing, while individual team members can be made recognisable by a back number on a shirt.
In hot weather garments should allow the wearer to stay cool. In cold weather garments should help to stay warm. In cold climates the best athletic wear should not only provide warmth but also transfer sweat away from the skin. For activities such as skiing and mountain climbing this is achieved by using layering: moisture transferring materials must be worn next to the skin, followed by an insulating layer, and wind and water resistant shell garments.