Many studies on evidence-based policy are still clinging to a linear model. Instead, we propose to understand expertise and evidence as 'socially embedded' in authority relations and cultural contexts. Policy-relevant facts are the result of an intensive and complex struggle for political and epistemic authority. This is especially true where science and policy are difficult to distinguish and the guidelines for validating knowledge are highly contested. To understand the mechanisms leading to policy-based evidence and the long-term consequences of these transformations more comparative research on the cultural and institutional 'embeddedness' of epistemic and political authority is needed.