Policy or program evaluation is rooted in the practices of contemporary public management. In this article, we describe the many aims pursued by evaluation (e.g. information production, program improvement, the fuelling of democratic debate) and the principal debates that drive the evaluation community. Among these debates, the one that focuses on the trend of evidence based policy is one of the more recent and the most lively. The principal tensions focus on the hierarchical organization of evaluative knowledge, the constrained use of knowledge and the will to homogenize evaluative practice. In observing these developments, we also notice a resurgence in the rationalization of decision-making processes and principles of public management that seems to be out of step with the aspirations of the population for participation and transparency. This article also presents the challenges confronting decision-makers and public administrators when faced with the reinvention of the rational state.