4 | 101 The transition to adulthood: A game changer!? 2013; Sarrazin et al., 2001; Scheerder et al., 2006). Activity levels and club sport participation are generally highest among youngsters (European Commission, 2014). Upon reaching adulthood many people quit club-organised sport (Borgers et al., 2016b; European Commission, 2014; Lunn, 2010; Pilgaard, 2013; Scheerder et al., 2006; Vandermeerschen et al., 2016). Findings on the relationship between age and sport activity, however, are contradictory (Borgers et al., 2016a). The issue is whether dropping out of a (club-organised) sport during the transition to adulthood means quitting sport altogether, or if it merely reflects a way of continuing sport under other conditions, like a different informal or organisational setting (Borgers et al., 2016b). In our study we analysed changes in individuals’ sport frequency and number of sports practised, alongside the likelihood of switching from a club sport to some other informal or organisational setting, or dropping out of sport altogether. This last aspect is particularly relevant, as sport contexts and practices have become increasingly diversified and de-traditionalised (Borgers et al., 2018; Klostermann & Nagel, 2014). Second, the current study expands on earlier research on sport participation by focusing onmultiple individual life events that mark the transition to adulthood. We examined five major life events: leaving full-time education, beginning to work, entering an intimate relationship, starting to cohabit or getting married, and the birth of the first child (Arnett, 2007; Kilmartin, 2000; Raymore et al., 2001; Zarrett & Eccles, 2006). These life events alter roles and responsibilities, leading to changes in daily routines and opportunities (Hirvensalo & Lintunen, 2011; Holmes & Rahe, 1967). Experiencing such a life event also changes needs, resources, and restrictions with regard to sport participation (Engel & Nagel, 2011; Hirvensalo & Lintunen, 2011; Lunn, 2010; Pilgaard, 2013; Van Houten et al., 2014, 2017). Few previous studies have examined the influence of such major life events on sport participation (Allender et al., 2008; Engberg et al., 2012; Hirvensalo & Lintunen, 2011). In our study, we developed a theoretical framework to explain the consequences of major life events for sport participation, based on changes in temporal and social resources associated with major life events (Kraaykamp et al., 2009; Schor, 1991; Van Houten et al., 2014, 2017). Third, we analytically propose a prospective life-course design (Engel & Nagel, 2011; Heikkinen, 2010; Hirvensalo & Lintunen, 2011) to test whether major life events affect an individual’s sport participation. Previous research has