This book offers an accessible introduction to the topic of impact evaluation and its practice in development. While the book is geared principally towards development practitioners and policymakers designing prospective impact evaluations, we trust that it will be a valuable resource for students and others interested in using impact evaluation. Prospective impact evaluations should be used selectively to assess whether or not a program has achieved its intended results, or to test alternatives for achieving those results. We consider that more and better impact evaluation will help strengthen the evidence base for development policies and programs around the world. If governments and development practitioners can make policy decisions based on evidence – including evidence generated through impact evaluation – our hope is that development resources will be spent more effectively, and ultimately have a greater impact on reducing poverty and improving people s lives.