The bicycle is a human-powered, two-wheeled vehicle that can be used for transport and recreation. There are currently over 1 billion bicycles in use worldwide. Bicycles can be used for racing, commuting to work or school, touring, physical fitness, and artistic expression. Cycling is an important mode of transportation in many countries. It is also a popular recreational activity, with participants ranging from children to competitive athletes. The basic design of the bicycle has changed little since the invention of the first chain-driven model in 1885. The bicycle can be adapted to carry cargo, a child seat, or both.

Bicycles are often the primary form of transportation in developing nations, as they offer a relatively affordable and efficient means to transport goods and people over long distances. In addition, the bicycle can be used to access areas that are not easily accessible by car or bus. Several companies in the United States, including Ford Motor Company and Harley-Davidson, started out as bicycle manufacturers. The modern bicycle is usually designed for comfort and efficiency, with features such as front suspension, disk brakes, and a wide range of gearing, providing a smooth, comfortable ride.

Most bicycles have a frame made of steel or aluminium, with one or more wheels. A crank mechanism, with a fixed chain and freewheel, is attached to the frame, allowing the rider to pedal the bicycle in either direction. The wheels may be solid, air-filled, or made of wire-spoked tyres.

Many models have pegs, which are fastened to the wheel hubs, that can be used to help perform tricks, allow a place for extra riders to stand, or to attach a trailer for carrying cargo. Parents frequently add rear-mounted child seats or auxiliary saddles fitted to the crossbar for transporting children.

Cycling can be a very healthy and effective exercise, increasing strength and stamina. However, it is important to increase duration and frequency gradually to prevent injury and overuse. Common causes of overuse are muscle soreness, knee pain, numbness, and imbalance. Occasionally, a shin splint or other joint pain may indicate the presence of an underlying medical condition.

Generally, the bicycle is easy to maintain, with most components able to be removed and replaced by the rider. Regular maintenance is necessary to keep the bicycle in good working order. Many cyclists are capable of performing most maintenance tasks themselves, aided by tools such as a wrench and socket set, although certain tasks, such as wheel removal and axle replacement, require special or manufacturer-specific tools.