Made up of three different disciplines; show-jumping, dressage and cross-country.
Show-Jumping sees horse and rider try to jump over between 12 and 15 fences in a sand or grass area against the clock. If they knock a fence down they are docked penalty points or time penalties.
Cross-country is similar but instead of an enclosed arena the fences are spread around a course that goes through the countryside. They have to get over 45 fences spread over almost 6,000m.
Dressage tests the rider and horse's ability to complete a series of moves and walks. They are marked by a set of judges. It's supposed to look like the horse is doing all the work on their own, without the rider telling them what to do.
All three disciplines are combined in the three-day eventing part of the competition.
Who are the British hopes for Athens?
Pippa Funnell is probably the best single hope for a medal, and won the three biggest events in the equestrian world in 2003 - at Kentucky, Badminton and Burghley.
She was also part of the eventing team that won silver at the Sydney Games, and the team should challenge for medals again this year.
Who are the big stars?
Ludger Beerbaum has been to four Olympic Games and has returned home to Germany with a gold medal every time.
He's won team gold three times and individual once, and will be the man the British riders have to beat if they want to win.