Obesity as a major public health and economic problem has risen to the top of policy and programme agendas in many countries, with prevention of childhood obesity providing a particularly compelling mandate for action. There is widespread agreement that action is needed urgently, that it should be comprehensive and sustained, and that it should be evidence-based. While policy and programme funding decisions are inevitably subject to a variety of historical, social, and political influences, a framework for defining their evidence base is needed. This paper describes the development of an evidence-based, decision-making framework that is particularly relevant to obesity prevention. Building upon existing work within the fields of public health and health promotion, the Prevention Group of the International Obesity Task Force (IOTF) developed a set of key issues and evidence requirements for obesity prevention. These were presented and discussed at an IOTF workshop in April 2004 and were then further developed into a practical framework. The framework is defined by five key policy and programme issues that form the basis of the framework. These are: (i) building a case for action on obesity; (ii) identifying contributing factors and points of intervention; (iii) defining the opportunities for action; (iv)evaluating potential interventions; and (v) selecting a portfolio of specific policies, programmes, and actions. Each issue has a different set of evidence requirements and analytical outputs to support policy and programme decision-making. Issue 4 was identified as currently the most problematic because of the relative lack of efficacy and effectiveness studies. Compared with clinical decision-making where the evidence base is dominated by randomized controlled trials with high internal validity, the evidence base for obesity prevention needs many different types of evidence and often needs the informed opinions of stakeholders to ensure external validity and contextual relevance.