Men race over a distance of 40-50 kilometres and the women over 30-40km - the first to the finish line wins.
Mountain biking can be a rough ride
The final decision on the length of the course is not made until the night before the race, to take into account weather conditions.
Competitors start at the same time and complete a lap of the 1.8km start loop before heading out onto the main course.
Unlike road and track cycling, where riders can receive assistance for mechanical troubles, mountain bikers cannot.
They carry their own repair kits and must see to every problem themselves.
The Sydney Olympic course four years ago was a bush track of mainly rough terrain, featuring gullies and other obstacles.
Mountain biking made its Olympic debut in Atlanta in 1996
Bikes are much sturdier than in the road and track events. They are fitted with a front suspension similar to that of a motorbike, and often have shock absorbers at the rear.
They have straight handlebars, powerful brakes and 24 gears.