Out of several years of research experience in the field of religion in prison, this article proposes some reflexions reflections on how to take into account both a superdiverse situation and power issues in public total institutions. It proposes to use the notion of ‘gray zone,’ which refers to both the fluidity of current religious practices in contrast to a clear-cut distinction between secularity and religion, and to an ambiguous positioning of actors in the complex field of institutional power relations. In a first step, the author discusses briefly the structural locations of religion in European public total institutions and the ways these have been studied. In a second step, the focus goes on those locations and uses of religion which are informal and which open up what the author calls a ‘gray zone.’ The concluding remarks highlight the complex power issues and relations at work in the realm of such a gray zone.