A bicycle, also called a bike or a pedal cycle, is a human-powered, two-wheeled, self-propelled vehicle with a frame and one or more axles. It is designed for riding on paved paths, dirt trails or unpaved surfaces such as gravel. A person using a bicycle is called a cyclist or a bicyclist. Bicycles have become one of the most important means of transportation in many parts of the world, as they are cheap, easy to maintain and environmentally friendly. They are used by individuals for recreation, transportation, commuting and as a means of freight transport.

The bicycle is the most efficient human-powered means of transportation in terms of the amount of energy a rider needs to expend for a given distance. This is due to the fact that up to 99% of the energy applied to the pedals is transferred to the wheels, making bicycles much more effective than walking or running. The bicycle has had a profound effect on human society, changing lifestyles and culture. It has allowed people to work and shop in cities without having to depend on horses or trains, and to travel for leisure at speeds three to four times greater than walking. It has also reduced crowding in inner-city tenements, and made it possible for people to commute into the countryside from suburbia.

Modern bicycles have a number of accessories, to make the riding experience more comfortable and convenient. These include safety equipment such as helmets, reflectors and lights; technical accessories such as dynamos, racks, cages, bags and water bottles; and apparel such as shoes, gloves and jerseys. Bicycles may also be equipped with special equipment for transporting children, such as child carriers and trailers.

Most bicycles require regular maintenance and periodic replacement of components such as tires, chains and bearings. The most common method of maintenance is cleaning and lubrication. Bicyclists can perform most of this work themselves, or have it done by a professional. Some components, such as the wheel axles and hubs, are more difficult to service.

Bicycles are relatively simple machines when compared to cars, so they have been widely adopted as an affordable mode of transportation in countries with developing economies. They are easy to maintain and can be very reliable when maintained properly. They are also relatively quiet, and have the advantage of not polluting the air. They also help to reduce traffic congestion and provide exercise for the riders. In China, the bicycle is so popular that Deng Xiaoping once described prosperity as having “a Flying Pigeon in every home.”