AbstractEvidence‐based policy‐making increasingly shapes the practice of advisory bodies, including global environmental assessments (GEAs). Advocates point to the power of evidence (particularly, but not only scientific evidence) to improve policy‐making. Here we discuss how political considerations shaped evidence‐gathering and use within the World Commission on Dams (WCD), a GEA body which was active between 1998 and 2000. We use insights from semi‐structured interviews with participants in the WCD process. First, we argue that the WCD shows that the political nature of evidence‐gathering has long been important in GEA processes. Despite rhetoric emphasising the objectivity of its evidence base, the WCD's evidence‐gathering was permeated by political considerations, for example in convening stakeholders with opposing views, giving evidence an instrumental purpose in widening participation and epistemic authority beyond just information and learning. Second, we show how a diversity of evidence (in form and substantive content) can challenge mainstream views. Contrary to the conventional emphasis on technical and quantitative data in GEA processes, we show how personal engagement with emotionally charged evidence, including that from grassroots sources and participatory processes within the WCD, created a shared understanding among opposing sides.