This article examines the reasons we need evidence for policy, discusses where evidence is needed in the policy-making process, and the nature of the evidence base for strategy and policy. Working relationships between policy makers and their advisers are key: as policy makers come from a variety of backgrounds, developing a common language helps set discussions about the robustness of the evidence base on a sound footing. The article identifies five components of robustness, proposes a series of questions that could be used to address them and discusses the implications for the processes of policy making.