Cycling is an activity most commonly performed on a bicycle - when it is it is also referred to as bicycling or simply biking. It is the use of the bicycle, unicycle (unicycling), tricycles (tricycling), quadracycles (quadracycling), and other similar wheeled human-powered vehicles (HPVs) for the purpose of transport, as a form of recreation, or in racing. It is done on roads and paths, across open country or even over snow and ice (icebiking).
Bicycles were introduced in the 19th century and now number about one billion worldwide. They are the principal means of transportation in many regions.
Bicycling is a highly efficient mode of transportation  and optimal for short to moderate distances. Compared to motor vehicles, bicycles have numerous benefits including the provision of exercise, generating renewable energy and thus no air pollution, reducing traffic congestion, minimizing noise pollution (nearly silent operation), easier and less costly parking, much lower likelihood of causing a fatality, high maneuverability, ability to travel on roads or special paths, and lower user cost as well as societal costs (negligible damage to roads, and less pavement required). Criticisms and downsides to cycling commonly include: reduced protection in crashes (including those with motor vehicles), longer travel time (except in densely populated areas), no inherent protection from poor weather, inability to transport passengers, and the physical demands of operation.
As a sport, cycling is governed internationally by the Union Cycliste Internationale in Switzerland (for upright bicycles) and by the International Human Powered Vehicle Association (for other HPVs, or human-powered vehicles). Cycling for transport and touring is promoted on a European level by the European Cyclists' Federation, with associated members from Great Britain, Japan and elsewhere. Regular conferences on cycling as transport are held under the auspices of Velo City; global conferences are coordinated by Velo Mondia
In many countries, the most commonly used vehicle for road transport is a utility bicycle. These have frames with relaxed geometry, protecting the rider from shocks from the road, and easing steering at low speeds.
Road bikes tend to have a more upright shape and a shorter wheelbase, which make the bike more mobile but harder to ride slowly. The design, coupled with low or dropped handlebars, requires the rider to bend forward more, utilizing stronger muscles and reducing air resistance at high speed.
The price of a new bicycle can range from US$50 to more than US$20,000, depending on quality, type and weight (the most exotic road bicycles can weigh as little as 3.2 kg (7 lb)). Being measured for a bike and taking it for a test ride are recommended before buying.
The drivetrain components of the bike should also be considered. A middle grade dérailleur is sufficient for a beginner, although many utility bikes come equipped with hub gears. If the rider plans a significant amount of hillclimbing, a triple-crank (three chainrings) front gear system may be preferred. Otherwise, the relatively lighter and less expensive two chainrings may be better.
Many road bikes along with mountain bikes include clipless pedals to which special shoes attach, via a cleat, permitting the rider to pull on the pedals as well as push. Other possible accessories for the bicycle include headlights and brake lights, bells or horns, disc brakes, child carrying seats, cycling computers with GPS, locks, bar tape, fenders, baggage racks, baggage carriers and pannier bags, water bottles and bottle cages.
For basic maintenance and repairs, cyclists can choose to carry a pump (or a CO2 cartridge), a puncture repair kit, a spare inner tube, and tire levers. Cycling can be more efficient and comfortable with special shoes, gloves, and shorts. In wet weather, riding can be more tolerable with waterproof clothes, such as cape, jacket, pants and overshoes.
Items legally required in some jurisdictions, or voluntarily adopted for safety reasons, include bicycle helmets, generator or battery operated lights, reflectors, and audible signaling devices such as a bell or horn. Extras include studded tires and a bicycle computer.
Bikes can also be heavily customized, with different seat designs and handle bars, for example. The British art critic and presenter of the South Bank show, Melvyn Bragg, is a keen cycling enthusiast having customised his stunt racing bike with handle bar tassles, tubeless tires, and even icons of the A team, which appear with a flick of a switch.
Main article: vehicular cycling
Learning to ride efficiently and safely in traffic is important. In the United Kingdom, many primary school children took the Cycling Proficiency Test, to help them cycle more safely. However, the Cycling Proficiency Test has now been superseded, for children, by 'Bikeability' and the National Standards for Cycle Training. In countries such as the Netherlands, where cycling is popular, cyclists sometimes ride in bike lanes at the side of, or separate from, main highways and roads. Many primary schools participate in the national road test in which children individually complete a circuit on roads near the school while being observed by testers.
The physical exercise gained from cycling is generally linked with increased health and well-being. According to the World Health Organization, physical inactivity is second only to tobacco smoking as a health risk in developed countries, and this is associated with many tens of billions of dollars of healthcare costs. The WHO's report suggests that increasing physical activity is a public health 'best buy', and that cycling is a 'highly suitable activity' for this purpose. The charity Sustrans reports that investment in cycling provision can give a 20:1 return from health and other benefits. It has been estimated that, on average, approximately 20 life-years are gained from the health benefits of road bicycling for every life-year lost through injury.
Bicycles are often used by people seeking to improve their fitness and cardiovascular health. In this regard, cycling is especially helpful for those with arthritis of the lower limbs who are unable to pursue sports that cause impact to the knees and other joints. Since cycling can be used for the practical purpose of transportation, there can be less need for self-discipline to exercise. Interestingly, it has been found that despite toning the leg muscles, cycling also tones the buttocks.
Cycling while seated is a relatively non-weight bearing exercise that, like swimming, does little to promote bone density. Cycling up and out of the saddle, on the other hand, does a better job by transferring more of the rider's body weight to the legs. However, excessive cycling while standing can cause knee damage. It used to be thought that cycling while standing was less energy efficient, but recent research has proven this not to be true. Other than air resistance, there is no wasted energy from cycling while standing if it is done correctly.
Cycling on a stationary cycle is frequently advocated as a suitable exercise for rehabilitation, particularly for lower limb injury due to the low impact that it has on the joints. In particular cycling is commonly used within knee rehabilitation programs.
As a response to the increased global sedentarity and consequent overweight and obesity, one response that has been adopted by many organizations concerned with health and environment is the promotion of Active travel, which seeks to promote walking and cycling as safe and attractive alternatives to motorized transport. Given that many journeys are for relatively short distances, there is considerable scope to replace car use with walking or cycling, though in many setting this may require some infrastructure modification, particularly to attract the less experienced and confident.
Cycling is seen by some to be an inherently high-risk, dangerous activity although use of appropriate safety equipment and obedience of road rules can reduce risk of serious injury. In the UK, fatality rates per mile or kilometre are slightly less than those for walking. In the US, bicycling fatality rates are less than 2/3 of those walking the same distance. For a child cyclist the rate per mile or kilometre travelled is around 55 times that for a child occupant of a car, while the fatality and serious injury rates per hour of travel are just over double for cycling than for walking (due to the reduced travel time), in the UK. It should be noted that calculated fatality rates based on distance for bicycling (as well as for walking) can have an exceptionally large margin of error, since there are generally no annual registrations or odometers required for bicycles (as there are with motor vehicles), and this means the distance traveled must be estimated.
Most cycle deaths result from a collision with a car or heavy goods vehicle, both motorist and cyclist have been found responsible for collisions  However, a very high proportion of non-fatal injuries to cyclists do not involve any other person or vehicle.
A Danish study in 2000 concluded that "bicycling to work decreased risk of mortality in approximately 40% after multivariate adjustment, including leisure time physical activity".  This conclusion is open to various interpretations.
Injuries (to cyclists, from cycling) can be divided into two types:
Physical trauma (extrinsic)
Acute physical trauma includes injuries to the head and extremities resulting from falls and collisions. Since a large percentage of the collisions between motor and pedal vehicles occur at night, bicycle lighting is required for safety when bicycling at night.
The most common cycling overuse injury occurs in the knees, affecting cyclists at all levels. These are caused by many factors:
Incorrect bicycle fit or adjustment, particularly the saddle.
Incorrect adjustment of clipless pedals.
Too many hills, or too many miles, too early in the training season.
Poor training preparation for long touring rides.
Selecting too high a gear. A lower gear for uphill climb protects the knees, even though your muscles are well able to handle a higher gear.
Excessive saddle height can cause posterior knee pain, while setting the saddle too low can cause pain in the anterior of the knee. An incorrectly fitted saddle may eventually lead to muscle imbalance. A 25 to 35 degree knee angle is recommended to avoid an overuse injury.
Overuse injuries, including chronic nerve damage at weight bearing ********s, can occur as a result of repeatedly riding a bicycle for extended periods of time. Damage to the ulnar nerve in the palm, carpal tunnel in the wrist, the genitourinary tract or bicycle seat neuropathy may result from overuse. Recumbent bicycles are designed on different ergonomic principles and eliminate pressure from the saddle and handlebars, due to the relaxed riding position.
Note that overuse is a relative term, and capacity varies greatly between individuals. Someone starting out in cycling must be careful to increase length and frequency of cycling sessions slowly, starting for example at an hour or two per day, or a hundred miles or kilometers per week. Muscular pain is a normal by-product of the training process, but joint pain and numbness are early signs of overuse injury.
Cycling has been linked to sexual impotence due to pressure on the perineum from the seat, but fitting a proper sized seat prevents this effect. In extreme cases, Pudendal Nerve Entrapment can be a source of intractable perineal pain. Some cyclists with induced pudendal nerve pressure neuropathy gained relief from improvements in saddle position and riding techniques.
The National Institute for Occupational Safety and Health (NIOSH) has investigated the potential health effects of prolonged bicycling in police bicycle patrol units, including the possibility that some bicycle saddles exert excessive pressure on the urogenital area of cyclists, restricting blood flow to the genitals. NIOSH is investigating whether saddles developed without protruding noses (which remove the pressure from the urogenital area) will alleviate any potential health problems. 
A Spanish study of top triathletes found those who cover more than 186 miles (300 km) a week on their bikes have less than 4% normal looking sperm.
Despite rumors to the contrary, there is no scientific evidence linking cycling with testicular cancer in men.
Andy Pruitt, director of the Boulder Center for Sports Medicine, wrote a book about diagnosing, treating, and preventing cycling-related injuries. Andy Pruitt's Complete Medical Guide for Cyclists
One concern often expressed (both by non-cyclists and some cyclists) is the thought that riding in traffic exposes the cyclist to higher levels of air pollution, especially if he travels on or along busy roads. This has been shown to be untrue, as the pollutant and irritant count within cars is consistently higher, (presumably because of limited circulation of air within the car and due to the air intake being directly in the stream of other traffic).