The question of why so many developers dedicate time and effort into contributing to Open Source Projects (OSP) is one of the most intriguing questions in OS research. Several preliminary studies have theorized about and empirically examined this challenging question. They suggest a variety of reasons to explain this phenomenon but mostly rely on self-determination theory, with its extrinsic-intrinsic dichotomy, to explain the heterogeneous and complex nature of motivation in OSS. This paper provides an alternative, theory-driven approach, where three different, yet complementary, theories of motivation are combined, to explain the participation and outcomes of developers in OSP. More specifically, our multi-theoretical framework is based upon, social exchange theory, goal-orientation and expectancy theory. An empirical test of the model is provided within the context of SourceForge.net. The results offer new theoretical and practical insights into developers’ motivation and how it affects their participation and outcomes.