The advice of scientists is critical in the big policy issues of our time from energy and climate to health and criminal justice. If policy making is to be evidence based then there must be a major role for scientists. But the relationship has great potential for conflict. Is it wrong to publicise scientific advice if it contradicts government policy? What is the dividing line between the role of the scientist and that of the policy maker? David Nutt has first hand experience of these issues. He is Professor of Neuropsychopharmacology at Imperial College and as Chair of the Advisory Council on the Misuse of Drugs was the government’s chief drug adviser until he was asked to resign by the Home Secretary in October 2009. He will talk about the relationship between scientists and policy makers and the role of scientists in evidence based policy making.