CONTEXT:Evidence to support government programs to improve public health often is weak. Recognition of this "knowledge gap" has led to calls for more and better evaluation, but decisions about priorities for evaluation also need to be addressed in regard to financial restraint. METHODS:Using England's Healthy Community Challenge Fund as a case study, this article presents a set of questions to stimulate and structure debate among researchers, funders, and policymakers and help make decisions about evaluation within and between complex public health interventions as they evolve from initial concept to dissemination of full-scale intervention packages. FINDINGS:This approach can be used to identify the types of knowledge that might be generated from any evaluation, given the strength of evidence available in response to each of five questions, and to support a more systematic consideration of resource allocation decisions, depending on the types of knowledge required. CONCLUSIONS:The principles of this approach may be generalizable, and should be tested and refined for other complex public health and wider social interventions.