This thesis seeks to understand how social capital develops within voluntary sports clubs. It adopts a micro-perspective to examine how social capital, defined as ‘the ability to secure resources by virtue of membership in social networks or larger social structures’ (Portes and Landolt, 2000: 532), emerges from social interaction within voluntary sports club contexts. The thesis is informed by a critical realist perspective that focuses on the underlying social mechanisms involved and how they operate differently for different groups and individuals in different circumstances. The empirical analysis, which represents the main contribution of the thesis, is based on three case studies of voluntary sports clubs in the UK. These case studies were carried out over 18 months and involved a number of in-depth interviews with members and organisers and periods of observation at each of the clubs. The analysis shows that members accessed a range of resources through the social ties they formed at the clubs. Interestingly, the analysis demonstrates that, as well as forming strong and weak ties, as standard network models would predict, members formed ‘compartmentally intimate’ ties at the clubs: strong ties that were domain-specific. The analysis reveals that several core mechanisms – reciprocity exchanges, enforceable trust, value introjection and bounded solidarity – operated at the clubs to develop social capital for members, but that these mechanisms were influenced in multiple ways by various elements of context. In particular, the analysis focuses on the socio-organisational context of the clubs and identifies several key elements, including the nature of the focal activity, the voluntariness of participation, the co-operative nature of engagement, the relative absence of hierarchy and the diversity of membership. Overall, the thesis provides support for an organisationally embedded view of social capital development and offers a rare example of critical realist research on social capital.