This report from the Network of Networks for Impact Evaluation (NONIE) offers critical guidelines for evaluating the impact of a project or program. Impact evaluations play a key role in the drive for better evidence on results and development effectiveness. They are particularly well suited to answer important questions about whether development interventions do or do not work, whether they make a difference, and how cost-effective they are. Consequently, they can help ensure that scarce resources are allocated where they can have the most developmental impact.