Triathletes must excel at swimming, cycling and running
The traithlon is a huge endurance test consisting of swimming, cycling and running.
The three disciplines are run back-to-back and the winner is the first athlete to finish.
There are four types of triathlon and the Olympics features the classic event which consists of a 1.5km swim, a 40km bike ride and a 10km run.
The event is contested by both men and women, and in Athens each field will consist of 50 athletes.
Men usually finish in the region of one hour and 50 minutes, with a world record of 1:39:50 set by Great Britain's Simon Lessing in 1996.
The women's record dates to the same Cleveland event in 1996 and is held by Jackie Gallagher of Australia who crossed the line in a time of 1:50:52.
As vital as the speed at which athletes complete each discipline is the changeover in the transtion areas where a dropped second can prove costly.
The best athletes complete their change between events within 10 seconds.
Swimming starts both events, with competitors diving into the water en masse. They can perform any swimming stroke but as speed is of the essence front crawl is the preferred option.
1.5k swim : 40k bike : 10k run
Sprint: 750m : 20k : 5k
Half: 1.9k : 90k : 21k
Ironman: 3.8k : 180k : 42.2k
On the cycling section, if an athlete suffers a flat tyre they are permitted to run with the bike to a tyre-change station, but otherwise they must remain in the saddle at all times.
Penalties are awarded if an athlete impedes another. If it occurs during the swim, the offender is made to stop for 30 seconds at the end of the leg before continuing.
During the cycling leg an official may warn a competitor with a yellow card, meaning the athlete must stop and await permission to continue, while two yellows result in a red and automatic disqualification.
The Olympic course is 25km south of Athens at Vouliagmeni, with the swimming taking place off the Oceanida beach.
The cycling consists of a five-lap circuit with one 820m climb which has an 18% grade in parts before three 3.3km laps in the run.