Impact evaluation aims to answer whether and to what extent a development intervention has delivered its intended effects, thus enabling evidence-based policy making. The desire for more hard evidence of the effectiveness of development interventions has fueled a growing interest in rigorous impact evaluation in the international development community. This report discusses the fundamental challenge of impact evaluation, which is to credibly attribute the impact, if any, to the intervention concerned. It then discusses the merits and limitations of various impact evaluation methods. It also presents a survey of recent applications of impact evaluation, focusing on the typical evaluation problems looked at, methods used, and key findings. The report includes six case studies and outlines practical steps in implementing an impact evaluation.