Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are cyclists or bicyclists. Apart from ordinary two-wheeled bicycles, cycling also includes riding unicycles, tricycles, quadracycles, and other similar human-powered vehicles (HPVs).

Bicycles were introduced in the 19th century and now number about one billion worldwide. They are the principal means of transportation in many regions.

Shortly after the introduction of bicycles, competitions developed independently in many parts of the world. Early races involving boneshaker style bicycles were predictably fraught with injuries. Large races became popular during the 1890s "Golden Age of Cycling", with events across Europe, and in the U.S. and Japan as well. At one point, almost every major city in the US had a velodrome or two for track racing events, however since the middle of the 20th century cycling has become a minority sport in the US whilst in Continental Europe it continues to be a major sport, particularly in France, Belgium, Italy and Spain. The most famous of all bicycle races is the Tour de France. This began in 1903, and continues to capture the attention of the sporting world.

In 1899, Mile-a-Minute Murphy became the first man to ride his bicycle a mile in under a minute, which he did by drafting a locomotive at New York's Long Island.

As the bicycle evolved its various forms, different racing formats developed. Road races may involve both team and individual competition, and are contested in various ways. They range from the one-day road race, criterium, and time trial to multi-stage events like the Tour de France and its sister events which make up cycling's Grand Tours. Recumbent bicycles were banned from bike races in 1934 after Marcel Berthet set a new hour record in his Velodyne streamliner (49.992 km on November 18, 1933). 

Track bicycles are used for track cycling in Velodromes, while cyclo-cross races are held on rugged outdoor terrain, which is performed on road, grass, and mud. Riders in cyclocross must get off their bikes at certain intervals and hop over barriers. Time trial races, another form of road racing require a rider to ride against the clock. Time trials can be performed as a team or as a single rider. Bikes are changed for time trial races, using aero bars. In the past decade, mountain bike racing has also reached international popularity and is even an Olympic sport.

Professional racing organizations place limitations on the bicycles that can be used in the races that they sanction. For example, the Union Cycliste Internationale, the governing body of international cycle sport (which sanctions races such as the Tour de France), decided in the late 1990s to create additional rules which prohibit racing bicycles weighing less than 6.8 kilograms (14.96 pounds). The UCI rules also effectively ban some bicycle frame innovations (such as the recumbent bicycle) by requiring a double triangle structure.