This paper is concerned with the challenges involved in the transformation of the prison into a performance-oriented accounting entity. It examines the implication of private sector accounting and consulting expertise in redefining prison values and prison performance, and it discusses consequences this had for definitions of risk and responsibility. The article shows how the reforms promoted individual responsibilisation whilst at the same time leading to a systemic decentring of Prison Service accountability. Prison managers and regulators came to be inserted into hierarchies of expertise and credibility shaped by quests for commensuration and auditability. Second, the reform attempts brought about a situation of institutional lock-in by contributing, as the outgoing HM Chief Inspector of Prisons Anne Owers has put it, to the creation of an inflated prison system “too big to fail, and too big to succeed”.