A bicycle is a human-powered, two-wheeled vehicle that can be used for recreation, transportation or work. Bicycles are the most efficient means of transport a person can use, in terms of the energy required to travel a given distance. The bicycle is also one of the most popular forms of recreation and has been adapted for many uses, including racing and artistic cycling. The International Organization for Standardization has a special technical committee, TC149, which deals with standards related to cycles and their parts.

The modern bicycle is a relatively simple machine. The frame is a truss consisting of two triangles: the front triangle, which includes the head tube and top tube; and the rear triangle, which has paired chain stays and seat tubes. The wheels are attached to the lugs on the ends of these tubes, which have a fixed width and axle diameter. The cranks, chain and saddle are usually mounted on the seat tube. A handlebar is usually mounted on the seat tube, with a lever or brakes on the end of the bar.

Pegs can be fastened to either side of the wheel hubs to allow a place for a rider to stand, or a child seat can be fitted on the crossbar for carrying cargo or children. A hitch can be attached to the rear of the frame for towing a trailer.

Modern bicycles are designed for comfort, efficiency and speed. Pedal-assist systems can be added to help the rider pedal with less effort. This is especially useful for older or injured cyclists, and people who wish to commute longer distances. The most efficient bicycles are designed for racing, with lightweight frames, narrow tires and drop handlebars.

A bicycle can be made using a variety of materials, and the components may vary greatly in cost and performance. The most expensive bikes may feature carbon fiber frames and components, which are lighter and more aerodynamic than steel and aluminum. The frame geometry, material and build quality set the stage for how the bike will function, and can affect its handling, ride quality, and aesthetics.

Despite the many advantages of cycling, accidents and injuries are common. Injuries may be minor, such as scrapes or abrasions, or major, such as fractured bones and traumatic brain injuries. Bicycles are particularly hazardous for young children, who may crash without having the motor skills and cognitive abilities to control a moving object.

Those who use a bicycle for transportation or work should wear a helmet to protect against head injuries. Cyclists should also wear bright clothing or reflective gear to increase their visibility, and be aware of the traffic laws in their area. It is important to be alert for pedestrians, cars, trucks, other bicycles, animals, and changes in road terrain. It is also recommended to carry a repair kit and have tools on hand in case of an emergency. Bicycles are relatively simple machines, so many users maintain their own bikes; repairs are often straightforward and can be done with basic tools.