This is a crucial book for international development researchers who want the lessons of their research converted into changes in government and aid agency policy. International policy making is extremely complex and little studied. This book reviews what we do already know and provides a conceptual framework for future research. Through four case studies, ranging from local to international and from theory to practice, it explores the vital importance of cultures and structures, people, places and timing. International development work attracts some of the most intelligent entrants to the world of public service, young people who want their commitment to the shaping of a better world to be reflected in the policies of the ministries and agencies they have joined. This book will help to explain to those young people, and to their leaders in the international development community, how to bring about the changes in policy that will promote a true pro-poor development, based on the evidence of what works.