While three sport athletic competitions had been held prior to the 1970s, the first triathlon that included a sequence of swim, bicycle, and run events over set distances took place in Mission Bay, California, in 1974. With its easy access to the Pacific Ocean and excellent bicycling opportunities, the Mission Bay event had grown as a natural extension from the local athletic training scene. The triathlon became a fast-growing international sport, especially after the longest of the triathlon-styled events, the Hawaii Ironman, achieved international prominence in the early 1980s.

The ever-expanding number of races spurred the growth of a corresponding triathlon bureaucracy. The sport's governing body in the United States, USA Triathlon, was founded in 1982, with the corresponding international federation, the International Triathlon Union (ITU), formed in 1989. The world wide popularity of the triathlon was confirmed when the sport was made an Olympic event for both men and women in 2000. The ITU has over 150 nations as members, and hundreds of participatory as well as elite-level triathlon competitions are held annually throughout the world.

The basic swim/bike/run pattern is essential to the triathlon; there are five recognized distance formats used in official triathlon competitions. The swim portion is contested over an open body of water, and the cycling and running sections utilize open roads or, less frequently in the run stage, cross-country courses. The first type of triathlon is more informal, an introductory level race where each of the three segments are short, so as to encourage participation in the sport. Introductory level triathlon events are sometimes held in conjunction with a longer-sanctioned triathlon competition. A typical entry level triathlon will consist of race segments comprised of a 250-yd swim (250 m), a 5-mi (8 km) bicycle segment, and a 2-mi (3 km) run.

The remaining four triathlon distances are sanctioned by the ITU and each is subject to the ITU rules regarding the nature of the race course in each segment and other similar regulations. The sprint triathlon is a 0.5-mi swim (800 m), a 13-mi cycle (21 km), and a 3.2-mi run (5 km). Sprint triathlons are attractive to athletes both as a progression from an introductory triathlon, as well as a training mechanism for more advanced competitors in which to work on their speed in each segment. The Olympic triathlon is the most popular international format, representing the standards used at the Olympic Games; it is a 0.9 mi swim (1.5 km), 24.8 mi cycle (40 km), and a 6.2 mi run (10 km).

The longest of the triathlon competitions are the Ironman events, which take their name from the original Hawaii Ironman competition. The full Iron-man competition is a grueling 2.4 mile (4 km) swim, 112 mile (180 km) cycle, and a marathon run of 26.2 miles (42.2 km). Where the Ironman originated, on the big island of Hawaii, the heat and prevailing winds are as demanding an environmental factor as a triathlete would ever likely face. The half Ironman event is precisely 50% the length of the Ironman: 1.2 mile (2 km) swim, 56 mile (90 km) bike, and 13.1 mile (21 km) run.

Triathlon training is limited by fitness level, not age or gender. While an elite triathlete is usually very strong in one of the disciplines (most commonly, swimming, the most technically demanding of the three sports, where 70% of all ITU champions had their competitive sports background), successful triathlon participation will be built primarily on an understanding of the principles of cross training. Triathlon training will combine the development of muscle balance, as all musculoskeletal groups within the body must function optimally.

Implicit in effective triathlon cross training will be the enhancement of the flexibility of the joints and muscles through a focused and consistent stretching program. The majority of injuries sustained in triathlon are overuse or repetitive strain injuries, particularly in the legs and shoulders, given the nature of the triathlon's three components. Effective and consistent stretching, and a corresponding greater range of motion, serve to protect from overuse conditions to a degree; strains and other minor muscle or tendon ailments are a natural risk of the sport.

With the rise in the international popularity of the triathlon, there have been a number of sport-specific technological developments designed to increase athletic performance. In 1984, the Timex Ironman, a relatively inexpensive and durable timepiece, was created to assist the triathlete in the digital management of the split times in all three segments of the race, coupled with elapsed time capability. Hydrodynamic neoprene wetsuits were also developed to provide both additional buoyancy to the athlete (within permitted ITU regulation), as well as reducing the effect of drag created by the water on the swimmer. These suits are also built with closures to permit the swimmers to make a speedy exit from the suit as they prepared to proceed to the cycling portion in the transition area.

The triathlon also sparked changes in the design of bicycles built for triathlon racing. The high mounted seat and lowered handlebars of the "tri bikes," with the addition of aerodynamic extensions from the bars, permit the cyclist to take an extremely low and aerodynamic position on the road. With gear shifts mounted at the end of the extension, the rider is not required to change body position as often during the course of the race. The rider gains the additional advantage of being able to rest the upper body against the longer handle bars, a significant benefit to the athlete after the conclusion of the swim portion and the resulting demands on the upper body.

SEE ALSO Cross training; Cycling; Environmental conditions and training; Ironman competitions.