This thesis focuses on the dynamic relationship between social context and proactivity, i.e., the self-initiated behaviour to bring about future-focused changes. Chapter 1 is a general introduction. Chapter 2 presents a comprehensive review of how social context affects proactivity through multilevel paths. Chapter 3 shows that individuals are likely to help and socially undermine the proactive coworker, due to social comparison and personal identification. Chapter 4 reveals that team proactive personality, as a social context factor, conditions the relationship between individual proactive personality and work performance. Thus, this thesis shows that social context affects, responds to, and conditions proactivity.