This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book describes the deficiency by shedding light on the role of the prosecutor through an international comparative lens. It presents four closely related main themes relating to the role of the modern prosecutor: the nature of the prosecutor's office, the role of the prosecutor in investigations, prosecutorial discretion and how it is exercised, and politicisation and accountability of prosecutors. The book explores the question of the nature and impact of the prosecutors' office and powers, both within the criminal justice systems in which they operate, and within the wider society. It considers the explanations and rationales that have been offered in different jurisdictions for these very different approaches to the role of prosecutors in investigations, and for changes in their role in particular jurisdictions over time. The book focuses on the exercise of prosecutorial discretion, particularly in charging, sentencing and diversion.