This book offers the first comprehensive history of the Office of the Police Ombudsman for Northern Ireland (OPONI) - the only global example of a fully independent, civilian controlled model of police complaints- covering its development, outcomes, and distinctive operational role. OPONI is considered as the global 'gold standard' for police complaints systems in the world. Created in November 2000, it is the only functioning example in the world of a fully independent, civilian controlled model of dealing with complaints against the police - in this case the Police Service of Northern Ireland (PSNI). While much research has been devoted to the reform of the former Royal Ulster Constabulary and the creation of the PSNI, comparatively little attention has been devoted to the evolution and radical reform of the police complaints and accountability system in Northern Ireland. This book seeks to fill that scholarly gap and examine the establishment of OPONI set within debates about police complaints in country across nearly five decades. Drawing on the broader police complaints and accountability literature, the book explores how OPONI 'fits' into the wider international context and what lessons and opportunities exist for other jurisdictions grappling with limited, evolving, or transformative agendas around police accountability. It also considers the challenges for delivering an effective and independent model of police accountability through the lens of OPONI as the global 'gold standard'. This book will be of interest to all policing scholars, particularly those engaged with issues of democratic policing, police complaints, accountability, and legitimacy.