Source: (2004) IN, George Mair,ed., What Matters in Probation?. Cullompton, Devon,UK: Willan Publishing. Pp. 34-54.David Smith begins this chapter with a survey of attitudes in the probation service since 1945 towards the effectiveness of its work: first naïve optimism; then naïve pessimism; and now cautious and rational optimism. Smith sets out to explore how far this new optimism – which is part and parcel of the What Works approach of the National Probation Service – can be rationally justified. However, Smith makes clear he is not debating whether the evidence of research results demonstrates or does not demonstrate the effectiveness of What Works. For one thing, results to date are not adequate to support the cognitive behavioral basis of What Works, he states. Yet more, Smith questions the validity of this positivist, experimental, evidence-based approach in general. That is, he raises questions about the nature of evidence itself – what counts as evidence, and what evidence can and cannot tell us about effective practice. Thus, Smith scrutinizes the very foundation of the What Works perspective.