In Britain, as elsewhere, over the past two or three decades there has been growing concern over the extent to which sport and physical activity are becoming increasingly rare features of contemporary youth lifestyles. One corollary of this growing concern with youth lifestyles has been the widespread acceptance of a number of common sense assumptions about the nature of young people's sporting and leisure lives. Notwithstanding these concerns, Coalter (2004: 79) has noted recently that, at present, much of the existing research on young people, sport and leisure has consistently failed to explain adequately or provide 'any clear understanding of sport's (and physical activity's) place in participants' lifestyles'. The central objective of this sociological study, therefore, was to enhance our understanding of the place of sport and physical activity in the lives of a sample of 15-16-year-olds, and of the relationships between various aspects of their lives. More specifically, the thesis reports upon data generated by questionnaires completed by 1,010 15-16-year-olds who attended six secondary schools in the north-west of England and one secondary school in the north-east of Wales, as well as focus groups conducted with a sub-sample of 153 of these young people. The findings revealed that for many 15-16-year-olds, participation in sport and particularly 'lifestyle activities', was an integral aspect of both their school and leisure lives. In school physical education (PE) and extra-curricular PE, young people's participation - which was significantly related to sex and school attended - was largely dominated by competitive team-based sports that are typically gendered and stereotypical. The data also indicated that although there were no significant school- or age-related differences in participation in leisure-sport and physical activity overall, more males than females participated in sport and physical activity in their leisure time. Males were also the more frequent weekly participants and spent more time doing so than females. In addition, the data revealed that the leisure-sport and physical activity repertoires of 15-16-year-olds were characterized by involvement in more informally organized sports and highly-individualized recreational 'lifestyle activities', as well as a small number of team sports that were played competitively. It was also clear that participation in leisure-sport and physical activity was part of young people's quest for generating sociability and excitement in the company of friends and because it enabled them to do what they wanted, when they wanted and with whom they wanted. For many young people, however, and particularly the more frequent participants, playing sport and doing physical activity was just one component in their generally busy and wide-ranging leisure lives, which did not prevent them from engaging simultaneously in more sedentary activities (such as prolonged TV viewing and playing computer games) and commercially-oriented leisure activities, as well as consuming legal and illegal drugs. In this regard, it is argued that it is only possible to understand adequately where sport and physical activity fit into the multi-dimensional lives of 15-16-year-olds by examining those lives 'in the round', and by locating young people within the various networks of relationships to which they have belonged in the past, and which they continue to form in the present.