Worldwide, adventure tourism is a trillion-dollar subsector of the multi-trillion-dollar commercial tourism industry. It has grown from small beginnings as a result of: increasing urbanization of human populations in the developed world; increased marketing of adventure experiences through various forms of mass entertainment; increased commercialization of outdoor recreation; and increased commercial opportunities for associated equipment, clothing and residential property sales. A wide variety of different outdoor activities are available as commercial adventure tourism products. The majority of adventure tourism products are short-duration, low-skill, low-priced, high-volume trips offered close to mainstream tourism destinations. Clients are either passive, as in tandem skydiving; or take part with limited training provided as part of the product, as in whitewater paddle-rafting. A smaller proportion of adventure tourism products are longer-duration, higher-priced trips in remote areas, available only to active participants with prior skills and specialist equipment. The psychology of participants and guides in adventure tourism and recreation has been examined quite extensively, perhaps more so than the structure of the commercial products. It appears that there are a wide variety of different motivations, some personal and internal, and some social and external. Risk management aspects of adventure tourism, and to a lesser extent environmental management aspects, have also received research attention. Overall, considering the economic scale of the adventure tourism subsector relative to the rest of the tourism industry and other industry sectors, it is as yet remarkably little-studied. There are many research opportunities remaining, whether it is viewed as an industry subsector or a category of human behaviour. As yet there are apparently only three academic books on adventure tourism. The most recent are Adventure Tourism (2006, CABI) and Adventure Tourism Management (2010, Elsevier), both written by Ralf Buckley. The former presents and analyses a large set of case studies; the latter addresses a range of different management aspects. Both contain bibliographies of previous literature.