A triathlon is an endurance sports event consisting of swimming, cycling and running over various distances. As a result, proficiency in swimming, cycling, and running alone is not sufficient to guarantee a triathlon athlete a competitive time; trained triathlon athletes have learned to race each stage in a way that preserves their energy and endurance for subsequent stages. In most modern triathlons, these events are placed back-to-back in immediate sequence and a competitor's official time includes the time required to "transition" between the individual legs of the race, including any time necessary for changing clothes and shoes.

Triathletes tend to be extraordinarily fit, and many amateur athletes choose triathlon specifically for its fitness benefits. Because all three events are endurance sports, nearly all of triathlon training is cardiovascular exercise. In addition, since triathletes must train for three different disciplines, they tend to have more balanced whole-body muscular development than pure cyclists or runners, whose training emphasizes only a subset of their musculature.