A bicycle is a two-wheeled vehicle propelled by foot pedals and steered by handlebars. It is the most efficient human-powered means of transportation, converting up to 99% of the energy put into the pedals into forward motion. Bicycles are used for recreation, fitness, long-distance touring, and racing. They are also the primary mode of transport in many regions. Bicycles are constructed in a wide variety of styles and configurations to suit many different riding needs and environments, with multiple types of gearing available for varying terrain and speeds.

The modern bicycle originated in the 19th century, largely as a tool for transportation and recreation. Its design is based on that of a German invention of 1817, the draisine or Lauf-maschine, by Karl Drais. The first bicycles were built with wooden wheels and a rigid frame, and later all-metal models became popular, particularly the so-called “boneshakers” of the 1860s, which had solid iron wheels without tires that created a very rough ride. In the 1890s, large races involving these boneshakers began to develop, and cycling became a global phenomenon.

There are now more than 1 billion bicycles in the world, and they are the most popular form of personal transport. They are used for recreational riding, including mountain biking and road cycling, and competitive events like racing and track competitions. They are also used for courier services, and they can be fitted with specialized carriers for transporting cargo or children. Bicycles can be modified with a range of accessories to customize them for different uses, including front and rear lights, reflectors, fenders (mud-guards), bells, racks, and horns, as well as a variety of specialized clothing and equipment.

In the United States, where bicycles are very popular, there are a wide variety of specialized bicycles for different purposes. Some are designed for children to ride, and some for adults. Other specialized bicycles are used for bicycle racing and training, for fitness and health, and as a vehicle for artistic cycling. Many cities and towns have bike-share programs, where people can rent a bicycle for short periods of time.

Some bicycles are designed for tricks and stunts, such as those used in freestyle bicycle riding. Others are designed to carry riders of unusual sizes or for off-road use. Bicycles can even be adapted to carry wheelchairs.

The terms cyclist and biker are often confused. Cyclist usually refers to someone who uses a bicycle for sport, exercise, or transportation, and is associated with a focus on physical fitness, endurance, and competitive cycling. Biker, on the other hand, is a more casual term that can also encompass these activities, but it may be used in a more informal context and may emphasize different aspects of bicycle riding.

A literature review is a type of research paper that surveys the current state of knowledge about a particular topic or field. It includes a summary and evaluation of the literature, including its strengths and weaknesses. A literature review is usually organized around ideas rather than the sources themselves, as an annotated bibliography would be.