Mountain biking is a sport which consists of riding bicycles off-road, often over rough terrain, using specially adapted mountain 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: cross country, trail riding, all mountain, downhill, freeride, street riding, dirt jumping and trials. The vast majority of mountain biking falls into the recreational XC and Trail Riding categories.

This individual sport requires endurance, core strength and balance, bike handling skills and self-reliance. XC type mountain biking generally requires a smaller range of skills but a higher level of fitness than other types of mountain biking. Advanced riders pursue steep technical descents and, in the case of freeriding, downhilling, and dirt jumping, aerial maneuvers off of specially constructed jumps and ramps.

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

Mountain biking differs to track or road cycling as it mostly takes place off-road. Despite the 'mountain' part of the term, most mountain biking doesn't actually take place in the mountains. Because of the differences between the types of cycling, mountain biking requires a specialist type of bike, known as a mountain bike. Generally, mountain biking takes place on rough terrain, which will usually be muddy and often contains rocks. This is why mountain bikers need a specialist all-terrain mountain bike, as they are far better suited to the challenging conditions.