A bicycle is a human-powered two-wheeled vehicle that can be used for transportation and recreation. Its use is widespread in the world, where it is usually cheaper than an automobile and often provides a healthier mode of transport. It is also popular for sport and exercise, as well as a means of transport in inner cities.

There are several types of bicycles, ranging from the recreational to the professional racing bike. Each type differs from others in technical characteristics of the bicycle parts, some movement specifications, formal specifications and performance. These differences make it possible to have defined usages that correspond with each bicycle type.

The first modern bicycles were invented in the early 19th century, when German inventor Karl Drais made a wooden frame with pedals connected to the rear wheel by a chain. His invention was not very practical, as it required great effort to turn the wheels and only allowed for a slow speed of travel.

By the 1860s, inventors had developed more advanced bicycles with metal frames and tires. This era was marked by the invention of the freewheel, which enabled the bicycle to spin while not being pedaled and to stop easily using a hand brake (earlier designs used a pedal that was connected directly to the steering front wheel, making cycling more difficult and dangerous). During this period the term “cycle” was also introduced to describe the entire machine.

Later on, more improvements were made. Scottish inventor John Boyd Dunlop re-invented the pneumatic tire, which made riding easier and safer by absorbing the shock of bumps. Also, the chain drive was improved, allowing the bicycle to move more smoothly and quickly. Finally, in the 1890s, French inventor Aime Olivier created a diagonal single-piece frame made of wrought iron, which was much stronger and more practical than the earlier wood or wrought steel frames. This invention was the most significant in the history of the bicycle, as it shifted its public perception from that of a dangerous toy for young gentlemen to a useful everyday transportation tool for men and women of all ages.

Currently, millions of people around the world ride bicycles as a form of recreation, exercise and transportation. Some individuals, especially in industrialized countries, ride their bicycles for work. For example, in many large car manufacturing plants, employees use bicycles to move rapidly from one department to another. Bicycles are also commonly used in the United States and other countries by emergency medical services personnel, who can maneuver in crowded urban areas more quickly than a standard ambulance.

Overuse injuries in bicycle users may include compression of the nerves in the wrist from gripping the handlebars; neck, shoulder and back pain from repetitive flexing of the upper body; hip pain from sliding the soft tissue of the pelvis over the underlying bone; and saddle sores (for both men and women) from pressure on the seat. In addition, there is a risk of accidents with motor vehicles when using a bicycle on the roadways.