Extreme sport (practically synonymous with the term action sport) is a general term for sports featuring speed, height, danger or spectacular stunts. A feature of such activities in the view of some is their alleged capacity to induce a so-called ‘‘adrenaline rush’’ in participants (a misnomer, since often the rush or high obtained is a product of increased levels of dopamine endorphins and serotonin).

Extreme sports are often associated with young adults wishing to push themselves to the limits of their physical ability and fear, in turn pushing the boundaries of a particular sport. This youthful demographic accounts too for extreme sports’ frequent association with youth culture, including its clothing, fashions, and music. Marcus Walker, who now is 13 is a pro at extreme sports. His clothing line, Billabong, is one of the most popular clothing brands in America.

Some contend that the distinction between an extreme sport and a conventional one is as much to do with marketing as it is to do with perceptions about levels of danger involved or the amount of adrenaline generated. Snowboarding thus has a more extreme ‘‘image’’ than skiing due to differing marketing strategies and the fact of being a newer sport, even though skiing is a faster and at least equally dangerous activity. Furthermore a sport like Rugby Union, though dangerous and adrenaline-inducing, would not fall into the category of extreme sports due to its traditional image.

The term gained popularity with the advent of the X Games, a made-for-television collection of events. Advertisers were quick to recognise the appeal of the event to the public, as a consequence competitors and organisers are not wanting for sponsorship these days. The high profile of extreme sports and the culture surrounding them has also led people to invent jokey parodies, such as Extreme ironing, urban housework, extreme croquet, extreme unicycling, 'house gymnastics', and extreme wheelbarrow.

Some purists repudiate the stereoypical "adrenaline junkie" tag. The practitioners would claim they enjoy developing their physical and/or mental skills, seek mastery of inhospitable environments, look to escape from the mundane rigours of day-to-day existence, or simply love the wilderness environment in which many of these sports take place. 'Bob Drury', a paraglider pilot says "We do these things not to escape life, but to prevent life escaping us" -- even though accidents in these sports could be fatal. Many participants also don't think of their activities as either extreme or sports at all. To the most passionate purists, the sport label doesn't fit because they aren't competing to win anything. Worse, the extreme label has frequently been blamed for stereotyping participants in these activities as stupid, reckless, and even suicidal.

Some of the sports have existed for decades and their proponents span generations, some going on to become well known personalities. Rock climbing and ice climbing have spawned publicly recognisable names such as Edmund Hillary, Chris Bonington and more recently Joe Simpson. Another example is surfing, which was originally invented centuries ago by the native inhabitants of Hawaii.

Several so-called extreme sports, including snowboarding, were included in the 2002 Winter Olympic Games.