A bicycle is a human-powered, two-wheeled, steerable machine that converts human power into mobility and speed. It is the most efficient means yet devised for converting human energy into transport. Bicycles are used for transportation, recreation, and sport (see cycling). Globally there are twice as many bicycles as automobiles, and in cities like Copenhagen, they outnumber cars three to one. Many people use bicycles as their primary mode of transport, saving money, time, and reducing pollution and traffic congestion.

There are many different types of bicycles, but the first step in choosing a bike is understanding your needs. This may involve speaking with your doctor, who can recommend a bicycle that is most appropriate for you and your health concerns. For example, comfort bikes are designed for slow, leisurely rides such as Sunday trips to the local market or beach boardwalk journeys to take in the scenery. They are also great for riders with back problems.

Hybrid bicycles are a great choice for those who commute to work or school. They are great for getting around the city and can handle most road surfaces without needing to have a suspension system. Some hybrids come with disc brakes, which are more effective for higher speeds, but rim brakes still work well and are the standard for most urban bicycles.

For longer trips, a bike that is specially built for this purpose is called a touring bicycle. These are built for more comfort, and have spaces for carrying gear such as camping equipment or extra clothing. Some touring bikes have racks on their frames, while others have baskets or panniers attached to the front.

Cycling is also a great form of exercise, providing cardiovascular fitness, muscle strength, and endurance. It can also help with weight loss, reduces stress, and improves mental health. It is also a fun way to explore new places.

A bicycle has been a major part of warfare, serving as a scout vehicle and a means of transporting soldiers to combat areas. It was a main method of transportation during the Second Boer War, and in World War I, both sides employed bicycle troops for reconnaissance and transporting supplies. During the 1937 invasion of China, Japan employed 50,000 bicycle troops. In modern times, the military has used bicycles to train its soldiers, and many countries now have extensive bicycle paths for civilians and their armed forces.

While not required in all jurisdictions, it is recommended that cyclists wear helmets, carry a picture ID, and make a list of important phone numbers and radio stations to consult for weather or other road conditions while on a bike ride. Other items commonly carried by cyclists include a bike computer, a map or GPS device, spare inner tubes, bike lights, and high visibility clothing.