In England and Wales, when probation officers are asked the question, does what you are doing work, probation officers understand the term working to mean reducing the rate of offending among the client population. There are grounds to believing that some things work better than other things. The attitudes behind the answers could be described as naïve optimism, naïve pessimism, and rational optimism. This chapter explores how far this optimism can be justified and provides some context for the assertions about the death and resurrection of rehabilitative optimism. The claims of positivist social science have led to the belief that the social world is predictable and therefore controllable and that it is therefore possible and desirable to use research findings as the justification for managerial prescriptions of particular kinds of practice. The process of accreditation of programs for the probation and prison service exemplifies the managerial faith that research can be used to identify effective programs. This chapter argues that this managerial faith is misplaced, because it rests on a misunderstanding of the nature of evidence in the social sciences. The universal assumptions of the accreditation model are ill-founded because they pay insufficient attention to contexts and processes, and treat as unproblematic the question of what contextual and processual features may have produced a worthwhile difference. Evidence that emerges over a longer term will be regarded as more reliable and useful than what the positivist tradition has produced. References