Mountain biking is a sport which consists of riding bicycles off-road, often over rough terrain, using specially adapted mountain bikes or hybrid/cross road bikes. Mountain bikes share similarities with other bikes, but incorporate features designed to enhance durability and performance in rough terrain.

Mountain biking can generally be broken down into multiple categories: XC cross country, Trail Riding, AM All Mountain, DH downhill, FR freeride, Urban trials/street riding and DJ Dirt Jumping. The vast majority of mountain biking falls into the recreational XC and Trail Riding categories.

A combination sport named mountain bike orienteering adds the skill of map navigation to mountain biking.

This individual sport requires endurance, core strength and balance, bike handling skills (time in the saddle) and self-reliance. Advanced riders pursue steep technical decents and, in the case of Free Riding and Dirt Jumping, aerial maneuvers off of specially constructed jumps and ramps.

It can be performed almost anywhere from a back yard to a gravel road, but the majority of mountain bikers ride off-road trails, whether country back roads, fire roads, or singletrack (narrow trails that wind through forests, mountains, deserts, or fields). There are aspects of mountain biking that are more similar to trail running than regular bicycling. Because riders are often far from civilization, there is a strong ethic of self-reliance in the sport. Riders learn to repair their broken bikes or flat tires to avoid being stranded miles from help. This reliance on survival skills accounts for the group dynamics of the sport. Club rides and other forms of group rides are common, especially on longer treks.