102 | Chapter 14 employedmainly cross-sectional data (Pilgaard, 2013). This makes it impossible to investigate individual changes over time or to draw conclusions in terms of causes and consequences (Kraaykamp et al., 2013; Pilgaard, 2013; Van Houten et al., 2017). We used panel data (2009 and 2013) from the Netherlands Longitudinal Lifecourse Study (Tolsma, Kraaykamp, Graaf, Kalmijn, & Monden, 2014) to identify differences between and changes within the life course of respondents (Pilgaard, 2013). Our main research question reads as follows: to what extent do major life events that mark the transition to adulthood affect (1) the number of different sports practised by individuals, (2) the frequency of sport participation, and (3) the switch from practising sport (mostly) in a club setting to practising sport in other informal or organisational settings, or to not practising sport at all? THEORETICAL FRAMEWORK A changing context of sport participation Different sport participation profiles and configurations of sport systems can be found throughout Europe (Breuer, Hoekman, Nagel, & Werff, 2015; Camy et al., 2004; Van Tuyckom, 2013). In most countries (especially in the Western and Eastern parts of Europe), nonetheless, sport participation has become diversified and de-traditionalised in recent decades, due to societal processes of individualisation, modernisation and informalisation (see, e.g., Borgers et al., 2018; Klostermann&Nagel, 2014; Van Ingen&Dekker, 2011). This has produced the emergence of commercial health and fitness centres, mass market sporting events, sport programmes hosted by municipalities or private companies, and sporting activities organised by people on their own or in informal groups (Borgers et al., 2018; Borgers et al., 2016b; Breuer, Hoekman, Nagel, & Werff, 2015; Klostermann & Nagel, 2014). A conceptual model by Borgers et al. (2016) classifies these various sport settings based on how “heavy”, or demanding, participation is, according to their formal organisational structures and the