The need for a credible, accessible and conceptually sound evidence base for public health has never been more pressing. It is now widely acknowledged that improving health involves a range of different activities by many players on many fronts. Government policy, set out in the White Paper, 'Saving Lives: Our Healthier Nation', identifies specific targets for reducing avoidable deaths from cancer, coronary heart disease and stroke, accidents and suicides. It sets broader objectives for improving health and reducing health inequalities. It also stresses the importance of individuals, local communities and national government working together to achieve these goals. The government has emphasised its commitment to 'evidence-based policy-making' and wants to link public investment to knowledge of 'what works'. However, it is notoriously difficult to assess the effectiveness of many forms of public health activity, especially those relating to the underlying causes of health and ill health. Conventional approaches to evidence are insufficient and unhelpful. There is not enough evidence of the types required, and what does exist is too narrowly defined and biased towards a medical model of public health. A common approach, which all parties can endorse, is needed. a new shared framework is proposed, within which a more inclusive and appropriate evidence base can be built up over time. [Summary]