Eventing is the ultimate test of horse and rider, the equestrian equivalent of the triathlon combining the three disciplines of dressage, show jumping and cross country to produce an overall winner. Successful event horses must be well schooled and obedient, possess courage and stamina, and be very sound.

In each phase, horse and rider incur penalty points, which are added together. The winner is the partnership with the least number of penalty points after the three phases.

The first test is dressage, which involves a set of compulsory movements in an arena 20 metres wide and 40 metres long (60 metres long at higher levels of competition). The test is judged by one or more judges who are looking for balance, rhythm, suppleness and obedience of the horse and its harmony with the rider. Movements are marked out of 10 and the total added together to produce a penalty score.

The show jumping phase is one round of jumping with a maximum time allowed. Penalties are incurred for fences knocked down, refusals, falls and exceeding the time allowed. The fences are not as big as those in top class show jumping classes but are nevertheless a test for horses that are not specialist show jumpers.

The third phase is the cross country over a course of natural obstacles. Penalties are incurred for refusals and falls and exceeding the time allowed. In one day events, all three phases take place on the same day dressage, followed by show jumping and then the cross country.

Horses earn points through placings at events and gradually move up through the grades as they accumulate points. In all competitions, qualification refers to the horse rather than the rider.In other words, an advanced grade horse with a novice rider can only compete in events open to advanced horses but a leading international rider can compete in novice events on a novice horse.
On average, it takes three years to train a horse from the novice stage to an international three day event horse. Unlike racing, where jockeys often sit on horses for the first time in the paddock before the race, success in eventing is generally the result of a partnership between horse and rider based on trust built-up over time and experience.

The sport is very egalitarian. Top riders compete in novice events against inexperienced amateurs and men and women compete on equal terms, even at international level. Prize money is relatively low compared with many other sports and the top riders rely on enthusiastic owners and sponsors to finance the cost of training and keeping the number of horses that are required to compete at the top level.