Chapter 3 investigates the evolution of institutional shareholder activism, focusing on how a diverse range of investors – including pension funds, asset managers, hedge funds, and index funds – have moved from passive holders to more assertive and strategic actors in corporate governance. It traces the theoretical roots of shareholder activism, from the exit-versus-voice framework to the economic constraints and tactical shifts that define modern engagement. Using a typology of firm-, portfolio-, and system-level activism, the chapter explores a broadened strategic repertoire – illustrating a shift from adversarial tactics to more collaborative, coalition-based engagement – and examines how activist goals have expanded from financial returns to encompass ESG and systemic concerns. Drawing on a hand-collected dataset of UK activism campaigns (2010–20), it maps evolving trends in actors, agendas, and tactics, and analyses how ownership concentration and investment horizon influence outcomes. The chapter concludes by situating shareholder activism within policy debates, contrasting ‘engaged’ and ‘transient’ investors, and reframing shareholder voice within a pluralistic model of stewardship.