Secret informers are often the biggest threat faced by underground rebel groups, which must respond to this challenge in order to survive. Using the IRA as a case-study, Penality in the Underground offers a systematic, in-depth analysis of this phenomenon, providing an empirical and theoretical account of the causes, forms, functions, and effects of the underground response to informers. While superficial media images tend to depict only ruthless killings, the book argues - using the lens of 'Punishment and Society' and drawing on rich interviews with IRA members and on archival sources - that groups such as the IRA develop complex systems of punishment and social control in their pursuit of informers. The book demonstrates how such systems are not only a mechanical response to a security problem, but are also shaped by other goals, risks, and imperatives, such as maintaining legitimacy, projecting a state-like image, and supporting governance efforts. This work thus identifies and explains some remarkable features of the IRA's pursuit of informers, such as the establishment of 'courts-martial', the granting of 'amnesties', the expansion of social control, the productive function of labelling 'treason' in asserting sovereignty, and the long-term consequences of the issue during transition out of conflict. By exploring the penal logics, practices, and discourses of armed rebel groups - engaged in direct struggle with the state agencies that normally carry out criminal justice - the book aims to expand the study of punishment and society and demonstrate its utility to the understanding of non-state actors.