Visit any developing country and you will find governments, international donors, NGOs, and corporations involved in a range of innovative activities to address the needs of the poor. Only a fraction of those that show promise at a localized level, however, will ever be replicated, expanded, and sustained to achieve a transformative impact. Learning how to expand the reach of proven interventions so that they help larger numbers of poor people - 'scaling up' - is a fundamental challenge facing the developing world. This book improves our understanding of how scaling up can be achieved and what the international community can do to support the process. Remarkably little is understood of how to design scalable projects, the impediments to reaching scale, or the most appropriate pathways for reaching that goal. To answer these questions, this book features a series of case studies drawn from both the public and private sectors to demonstrate how the scaling up of services for the world's poor can happen. By linking public and private experience, the authors argue that successful scaling up will not be achieved by either public or private sector efforts alone. Rather, it will require both public and private efforts working together. This book demonstrates that the challenges to scaling up are complex and various, but ultimately surmountable. It provides an invaluable resource for development practitioners, analysts, and students on a topic that remains largely unexplored and poorly understood.