A bicycle is a human-powered vehicle that uses pedals to convert leg power into forward motion. There are many different types of bicycles, but they can generally be classified by their general construction, purpose or means of propulsion. Some common types of bicycles include utility bicycles, racing bicycles, touring bicycles, and hybrid bicycles. Some people also use unicycles and tricycles, which are technically not bicycles, but are often referred to informally as bikes.

The bicycle is the most efficient means of transportation in terms of the energy a person expends to travel a given distance, and has the advantage over other forms of human-powered vehicles of being capable of carrying a significant amount of cargo. Bicycles are also relatively easy to learn to operate, and can be built to be very reliable.

Modern bicycles have a variety of accessories that can enhance comfort and performance, such as suspension systems, which can absorb some of the shock from rough surfaces or terrain. They can also be equipped with racks and carriers for transporting cargo, such as bags or boxes. In addition, specialized bicycle helmets are available for cyclists who want to reduce the risk of injury.

Throughout history, the bicycle has had a profound impact on society and culture, especially in industrialized countries. Around the turn of the 20th century, it allowed workers to escape from crowding inner-city tenements and commute to suburban jobs; it reduced dependence on horses for transportation. It also enabled leisure activities such as jogging and skating, as well as competitive sports like road racing and freestyle BMX.

It is not known who first invented the bicycle, but a key improvement was made in 1868 by Pierre Lallement, who applied the pedals to the front wheel of a velocipede de pedale. This was a major step forward as it allowed the rider to both steer and use his/her legs to propel the bike. Eventually, larger wheels were used and chain drives were introduced to transmit the pedal power to the rear wheel.

The modern bicycle was further developed in the 1890s by Englishman J. K. Starley, who created the Rover bicycle in Coventry. This was the first recognizably modern design, with a frame with down tube, seat tube and top tube. Later, the chain drive was eliminated in favor of an efficient gear system, and the cranks were moved from the hubs to the frame to increase efficiency even further.

Today, there are a wide range of bicycles available to satisfy almost any need and budget. For example, there are entry-level models for casual riding that cost less than $1,000, and high-end racing bicycles that can easily exceed $15,000. Many manufacturers produce spare parts for bicycles, and a variety of technical accessories are available for cyclists to enhance their experience or performance. Some of these accessories include cyclocomputers for tracking speed, mileage, and heart rate, and bicycle lights to help drivers be seen more clearly at night.