This article examines the timorous courtship between public health law and evidence-based policy. Legislation, in the form of direct prescriptions or proscriptions on behaviour, is perhaps the most powerful tool available to the public health policymaker. Increasingly, the same policymakers have striven to ensure that interventions are based soundly on a secure evidence base. The modern mantra is that the policies to follow are the ones that have been demonstrated to work. Legislative interventions, involving trade-offs between public benefit and private interests, present formidable challenges for the evaluator. Systematic reviews of their overall efficacy, the main tool of evidence-based policy, are in their infancy. The article presents a design for such reviews using the example of a forthcoming synthesis on the effectiveness of banning smoking in cars carrying children.