Bicycles were introduced in the 19th century and now number about one billion worldwide, twice as many as motorcars. They are the principal means of transportation in many regions. They also provide a popular form of recreation, and have been adapted for such uses as children's toys, adult fitness, military and police applications, courier services, and competitive sports.

The basic shape and configuration of a typical bicycle has changed little since the first chain-driven model was developed around 1885. Many details have been improved, especially since the advent of modern materials and computer-aided design. These have allowed for a proliferation of specialized designs for particular types of cycling.

The invention of the bicycle has had an enormous impact on society, both in terms of culture and of advancing modern industrial production. Several components that eventually played a key role in the development of the automobile were originally invented for the bicycle - e.g., ball bearings, pneumatic tires, chain-driven sprockets, spoke-tensioned wheels, paved roads (replacing cobblestone) etc.

Uses:

Bicycles have been and are employed for many uses:

• Utility: bicycle commuting and utility cycling
• Work: mail delivery, paramedics, police, and general delivery.
• Recreation: bicycle touring, mountain biking, BMX and physical fitness.
• Racing: track racing, criterium, roller racing and time trial to multi-stage events like the Tour of California, Giro d'Italia, the Tour de France, the Vuelta a España, the Volta a Portugal, among others.
• Military: scouting, troop movement, supply of provisions, and patrol. See bicycle infantry.
• Show: entertainment and performance, e.g. circus clowns. Used as instrument by Frank Zappa.

Types:

Bicycles can be categorized in different ways: e.g. by function, by number of riders, by general construction, by gearing or by means of propulsion. The more common types include utility bicycles, mountain bicycles, racing bicycles, touring bicycles, hybrid bicycles, cruiser bicycles, and BMX bicycles. Less common are tandems, lowriders, tall bikes, fixed gear (fixed-wheel), folding models and recumbents (one of which was used to set the IHPVA Hour record).

Unicycles, tricycles and quadracycles are not strictly bicycles, as they have respectively one, three and four wheels, but are often referred to informally as "bikes".

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