The three-day event is an individual and a team competition
Riders compete for individual and team three-day event medals at the Olympics.
To win the three-day event, rider and horse must excel in three different discliplines - dressage, cross country and showjumping.
The winner is the rider or the team with the least penalty points at the end of the competition.
Both competitions will be held together in Athens, with an additional round of showjumping after team medals are decided to determine individual placings.
A nation may enter five riders in a team but only the best three scores count.
Should there be a tie at the end of the competition, the rider or team with the best cross-country score is the winner.
The first day takes the form of a dressage competition, and the climax is the showjumping.
In between is the most gruelling test, the cross-country.
This consists of a 5.2-kilometre course with a maximum of 45 "jumping efforts", with a double fence counting as two efforts.
Some of the jumps are more than a metre high and include perilous routes over water, ditches and banks, though there is usually an easier but longer route round.
Competitors who make it to the final event have to negotiate 10 to 12 obstacles in the showjumping.
It is easier than the showjumping medal event, but tough on a horse that has completed the arduous cross-country section just the day before.
Scoring for the three-day event is as follows:
Dressage - Riders must perform a set of 20 moves and are marked by judges for each move, how they control the horse and the obedience, pace and control of the animal.
They are penalised for each error. Points are converted into a penalty point score ready to add the penalties incurred in the next phases.
Cross-country - Penalty points are awarded for every second over the time limit.
Twenty points are awarded if the horse refuses to jump an obstacle, a second refusal at the same fence costs 40 points and a third means elimination.
A fall for the rider costs 65 points, but a fall for the horse means automatic elimination. The horse is considered to have fallen if his quarters touch the ground.
Showjumping - Knocking down a fence, or a refusal, costs four penalty points; a second refusal eight points. A third refusal means elimination.
A fall for the rider also costs eight points.