Drawing on Foucauldian genealogy, the article maps major sources and trajectories of the evidence discourse. This enables scrutiny of the current struggle about "evidence" for "What Works" in education and social welfare. Evidence discourse is identified as emerging from the medical field as a bottom-up professional strategy. It is subsequently reworked and launched into education and social welfare in moves that largely bypass professionals to serve policy-maker and market needs to enable evidence-based choices among public services. From this perspective, the author argues that education and social welfare professions may profit from adopting evidence as a floating signifier. An analytical distinction between external and internal forms of evidence is introduced to facilitate alternative strategies to dealing with the evidence discourse. (Contains 2 footnotes.)