Multisport is a family of athletic competitions in which athletes race in a continuous series of stages or "legs", and rapidly switch from one athletic discipline to another in order to achieve the best overall time. Most multisport events are endurance races, consisting of aerobic activities such as cycling, running, and cross-country skiing.

Duathlon is an athletic event (not to be confused with biathlon) that consists of a running leg, followed by a cycling leg and then another running leg in a format bearing some resemblance to triathlons. In the United States, Duathlon is governed by USA Triathlon; the International Triathlon Union governs the sport internationally.

An Ironman Triathlon is a triathlon sanctioned by the World Triathlon Corporation that adheres to specific distances for each part of the event. The Ironman World Championship is an annual triathlon race, possibly the most famous triathlon in the world. It is held in the US state of Hawaii. It features three endurance events: swimming, biking, and running. It is based in Kailua-Kona, Hawaii and involves a 2.4 mile (3.86 kilometre) swim (across Kailua-Kona Bay), followed by a 112 mile (180.2 kilometre) bike ride (from Keauhou to Hawi and back), and a 26 7/32 mile (42.195 kilometre) marathon along the coast of the Big Island (from Keauhou to Keahole Point to Kailua-Kona).

Official qualifying events for the Hawaii Ironman take place annually around the world, in places such as Australia, Canada, the United States, Japan, the Canary Islands (Spain), South Africa, and Europe.

Many people refer to all triathlons of this length as "Ironman" races, but this is technically incorrect. Race directors are careful to use generic terms, such as Iron Distance Triathlon, to describe races that are not affiliated with the World Triathlon Corporation, since "Ironman"® is a registered trademark, and can be used only with permission from the World Triathlon Corporation.

An Iron Distance Triathlon is a triathlon performed over the following distances:

* 3.8 km (2.4 mi) swimming
* 180 km (112 mi) bicycling
* 42.2 km (26.2 mi) running (the length of a standard marathon)

The historically first and most well known example is the Ironman Triathlon in Hawaii. The term "Ironman" is trademarked by the World Triathlon Corporation (WTC), which runs a series of races under that name. The ITU organizes a series of alternative races called Long Distance Triathlons. The distances for these races are close to but slightly different from the specifications for the Ironman Triathlon. An event that covers the same distance as an Ironman Triathlon is referred to as an Iron Distance Triathlon. Dozens of these independent (non-WTC)events are held each year throughout the world. The independent iron-distance triathlons are not qualifiers for the Hawaii Ironman Triathlon Championship race.

A variety of double, triple, quadruple, quintuple, and deca (10x) iron distance events are also held each year. In a deca iron distance event, for example, a 38 km swim is followed by an 1800 km bike race and a 422 km run. Astrid Benöhr holds the women's world records over the triple, quadruple and quintuple iron distances.

Off road triathalon is a form of triathlon, or three-stage race, consisting of a swim stage, mountain-biking stage, and a trail-running stage.

Off-road triathlons are distinguished from conventional triathlons in that the terrain for the cycling and running stages are generally unpaved, rough, and very steep and hilly. They require different techniques than conventional triathlon races, and in particular the athletes employ mountain bikes rather than road bikes.

An off-road triathlon requires a higher degree of technical biking skill, as opposed to the high speed and endurance demands of road biking in a road triathlon. Distances for the bike portion of an off-road are much less relevant than for a road triathlon. An off-road bike course may have several severe vertical climbs and descents. It may also have a high degree of technicality, meaning the number, pitch and sharpness of turns through trees, rocks, logs, streams and other obstacles on the bike trail. Because the vertical climbs and technical demands of an off-road course greatly reduce a biker's speed, and because the amount of climbing and technicality vary greatly from one off-road course to another, estimating times for an off-road bike course merely by distance is not reliable.

Off-road triathlon swim courses are normally similar to those of road triathlons. Each type of triathlon usually requires competitors to swim a minimum of 800 meters and typically 1500 meters in a lake, river or ocean.

Quadrathlon is an endurance sports event composed of four individual disciplines. All four disciplines are completed in succession and the lowest overall time decides the winner.

The World Quadrathlon Federation defines that a quadrathlon consists of

1. swimming
2. kayaking
3. cycling
4. running

in the above order.

However, other combinations, for example with roller skating or mountain biking, are also common
Off-road run courses often follow part of the off-road bike course, and so often require trail running up and down hills or mountain sides, through forests, streams, riverbeds and other natural and occasionally man-made obstacles. The run distance is at least 5 kilometers and normally 10 kilometers. Again, as in off-road biking, predicting a finish time purely based on distance will not be accurate, due to the climbing and trail turns and obstacles preventing a runner from reaching speeds usually achieved on the road.

The XTERRA is the best-known series and championship of off-road triathlons, although others exist.

In 1998, a double deca triathlon (20x iron distance) was held in Monterrey, Mexico. The winner, Vidmantas Urbonas of Lithuania, completed the race in 437 hrs 21 min 40 sec.

All of these various competitive events offer the serious athlete a way to challenge themselves and make their better.. best.