Martin Johnsrud Sundby is a former Norwegian cross-country skier who competed between 2003 and 2021. He is a two time Olympic champion at the 2018 Winter Olympics in the team sprint and relay and was also a silver and bronze medalist in the 30 km skiathlon in 2014 and 2018. Sundby is a 4-time world champion, winning his sole individual gold medal at the 15 km at the 2019 Nordic World Ski Championships in Seefeld. In 2014, he became the first Norwegian to win the Tour de Ski, a feat he repeated in 2016. He also won the overall world cup in 2014, 2016 and 2017.

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.