This report provides a guide to evaluating the impact of a project or programme. Impact evaluation is about attributing impacts to interventions, and can play a key role in development effectiveness. No single analytical method is best for addressing all aspects of impact evaluations, but some methods have an advantage over others in addressing a particular question or objective. Different methods can complement each other to provide a more complete picture of impact. Impact evaluations provide the greatest value when there is an articulated need to obtain the information they generate.