This paper examines how the concept of the 'evidence-based' approach has transferred from clinical medicine to public health and has been applied to health promotion and policy making. In policy making evidence has always been interpreted broadly to cover all types of reasoned enquiry and after some debate the same is now true for health promotion. Taking communities rather than individuals as the unit of intervention and the importance of context means that frequently randomized controlled trials are not appropriate for study of public health interventions. Further, the notion of a 'best solution' ignores the complexity of the decision making process. Evidence 'enlightens' policy makers shaping how policy problems are framed rather than providing the answer to any particular problem. There are lessons from the way that evidence-based policy is being applied in public health that could usefully be taken back into medicine.