The chapter introduces the central ideas of the book. It argues that the whistleblower is a key character in our democratic histories. Since whistleblowers are connected to notions of suspicion, disclosure, and visibility, they share several traits with activists and journalists. At the same time, they are sui generis because they act from an insider position, as organization members or employees. Such a sociological, historical, and democratic understanding of the whistleblower has gone relatively unnoticed in the literature. Asking these ‘big’ sociological questions gives us new tools for analysis. This is important because whistleblowing is gaining in prominence and impact in these years. The chapter offers a definition of the concept of whistleblowing and presents the different sociological thinkers that constitute the book’s theoretical foundations.