This paper discusses how two major constraints impact the practices and perceptions of teachers in French prisons: (i) the burden of penal and prison constraints and (ii) the splintering of professional teaching rationales. This paper examines the stakes of (i) a sociology of “third-party” actors in the prison setting; (ii) a sociology of the prison renewed by the approach of the decentralized study; and (iii) a sociology of professions, discourse, and professional segments.