Since the 1970s, numerous prisoners’ unions, often short-lived, have emerged in Western countries. This article, based on a study of prisoners’ unions in France and the United States since the 1970s, describes their various political trajectories, their strategies, and their organization. The authors discuss the prisoners’ unions in terms of prisoners’ movements, but also the many structures that may organize prisoners’ participation, such as “Inmates Councils”. Analyzing the common tactical dilemmas faced by prisoners’ unions, the authors thus define them as “unlikely organizations.”