The Western States Endurance Run, known commonly as the Western States 100 or Western States, is a 100.2-mile (161 km) ultramarathon that takes place on California's Sierra Nevada Mountains trails each year on the last full weekend of June. Western States is the oldest and one of the most competitive ultramarathons in the United States, and is administered by the nonprofit Western States Endurance Run Foundation.

An ultramarathon, also called ultra distance or ultra running, is a footrace longer than the traditional marathon length of 42.195 kilometres. Various distances, surfaces, and formats are raced competitively, from the shortest common ultramarathon of 31 miles (50 km) and up to 3100 miles. World Championships are held by the International Association of Ultrarunners (IAU) for 50 km, 100 km, 24 hours, and ultra trail running. The Global Organization of Multi-Day Ultramarathoners (GOMU) holds World Championships for 48 hours and 6 days. World Records are ratified and recognized by World Athletics, the IAU, and by GOMU.