Mountain bikes, also known as all-terrain bicycles or MTBs, are bicycles especially designed to ride on natural trails, which can include anything from unpaved roads to rocky tracks. The main difference between mountain bikes and standard road bikes is in the tires. As a rule, mountain bikes have thick tires and front-wheel suspension and offer advanced shock absorption. Some mountain bikes have rear suspension, also known as "soft tail," which allows the rider to move the wheel on pivots.

Mountain bikes can be divided into categories according to the terrain they are used on. There are basically eight categories of mountain bikes: cross country, all mountain, freeride, downhill, trials, dirt jumping, single-speed, and mountain cross. Cross country mountain bikes are the basic model from which most of the other originated. They are light weight, have little to no suspension, and are best for uphill or flat terrain. Trial mountain bikes are similar, except that they are used mostly in dirty trail competitions, rather than for day-to-day riding. Downhill and dirt jumping mountain bikes are geared to specific terrains. They are usually very strong bikes with excellent suspension, fast-rolling, and extremely durable.

The newest models to hit the market include all-mountain bikes, freeride, and single speed. These mountain bikes are designed for a specific purpose, and require superior skill and physical fitness. All-mountain bikes, for example, tend to be heavier than other models, and are used by riders who want to combine uphill and downhill riding with advanced suspension technology.

Most mountain bikes have 26-inch wheels, although 29-inchers are now becoming popular. Some riders, especially experienced ones, outfit their own bikes, adding suspension forks, changing chainstays, and taking off their seats, which is often not needed, as riders spend most of their time standing up on the pedals. Mountain bikes range in price from a few hundred dollars for a basic model to a few thousand for a professional version to be used in competitions. Mass-produced mountain bikes are widely available, and are usually enough for the occasional weekend rider.