From the Spanish Inquisition to Nazi Germany to the United States today, ordinary people have often chosen to turn in their neighbors to the authorities. What motivates citizens to inform on the people next door? In Judge Thy Neighbor, Patrick Bergemann provides a theoretical framework for understanding the motives for denunciations in terms of institutional structures and incentives. In case studies of societies in which denunciations were widespread, Bergemann merges historical and quantitative analysis to explore individual reasons for participation. He sheds light on Jewish converts’ shifting motives during the Spanish Inquisition; when and why seventeenth-century Romanov subjects fulfilled their obligation to report insults to the tsar’s honor; and the widespread petty and false complaints filed by German citizens under the Third Reich, as well as present-day plea bargains, whistleblowing, and crime reporting. Bergemann finds that when authorities use coercion or positive incentives to elicit information, individuals denounce out of self-preservation or to gain rewards. However, in the absence of these incentives, denunciations are often motivated by personal resentments and grudges. In both cases, denunciations facilitate social control not because of citizen loyalty or moral outrage but through the local interests of ordinary participants. Offering an empirically and theoretically rich account of the dynamics of denunciation as well as vivid descriptions of the denounced, Judge Thy Neighbor is a timely and compelling analysis of the reasons people turn in their acquaintances, with relevance beyond conventionally repressive regimes.