100 | Chapter 14 INTRODUCTION The social significance of sport has surged in recent decades. Appreciation of sport’s functional power stems from its advantageous consequences (Schlesinger & Nagel, 2015; Waardenburg & Van Bottenburg, 2013) in improving health (Miles, 2007), nurturing social capital and integration and thereby fostering social networks (Knoppers, 2006; Putnam, 1995; Seippel, 2006), and spurring economic growth (European Commission, 2013; Van der Meulen et al., 2012). Sport clubs form the heart of the Western European sport industry. Sport participation in club-organised settings is often regarded as especially beneficial (Borgers et al., 2016b; Breuer, Hoekman, Nagel, & Werff, 2015; Janssens & Verweel, 2014), especially for young people (Theeboom et al., 2010; Vandermeerschen et al., 2016), contributing to socialisation and enhancing social cohesion in particular. Accordingly, sport clubs have become increasingly important on the political agenda (Schlesinger & Nagel, 2015). “Sport for All” policies and programmes have been implemented at the local, national, and international level to lower the threshold for sport participation and boost involvement in sport, particularly club-organised sport, among people of all ages (DaCosta & Miragaya, 2002; European Commission, 2011; NOC*NSF, 2012; Tuyckom, 2011; Waardenburg & Van Bottenburg, 2013). Nonetheless, research shows that continuation (also called “tracking”) of sport participation from adolescence into adulthood remains a stumbling block (Hirvensalo & Lintunen, 2011; Malina, 2001; Telama, 2009; Vanreusel et al., 1997). Especially in late adolescence, individual sport participation levels decline (Engel & Nagel, 2011; Leslie et al., 2001; Van Tuyckom, 2011). Many adolescents drop out of club-organised sport (Borgers et al., 2016b; European Commission, 2014; Lunn, 2010; Pilgaard, 2013; Scheerder et al., 2006). The current study sought to better understand changes in sport participation during the transition to adulthood, pursuing several advancements. First, to bring out the dynamic character of sport participation, we investigated withinperson changes and differences between individuals in sport frequency, the number of sports practised, and the settings of sport activity during the transition to adulthood. Previous retrospective and longitudinal studies found that sport participation over the life course is characterised more by changes in activity level and sort of activities than by dropping out of sport with increasing age (Butcher et al., 2002; Engel & Nagel, 2011; Lunn, 2010; Pilgaard,