Competitive cross-country skiing encompasses a variety of race formats and course lengths. Rules of cross-country skiing are sanctioned by the International Ski Federation and by various national organizations. International competitions include the FIS Nordic World Ski Championships, the FIS Cross-Country World Cup, and at the Winter Olympic Games. Such races occur over homologated, groomed courses designed to support classic (in-track) and freestyle events, where the skiers may employ skate skiing. It also encompasses cross-country ski marathon events, sanctioned by the Worldloppet Ski Federation, and cross-country ski orienteering events, sanctioned by the International Orienteering Federation. Related forms of competition are biathlon, where competitors race on cross-country skis and stop to shoot at targets with rifles, and paralympic cross-country skiing that allows athletes with disabilities to compete at cross-country skiing with adaptive equipment.