Sarah Sunn Bush is Associate Professor of Political Science at the University of Pennsylvania. She is the author of two previous books with Cambridge University Press: Monitors and Meddlers: How Foreign Actors Influence Local Trust in Elections (2022) and The Taming of Democracy Assistance: Why Democracy Promotion Does Not Confront Dictators (2015). Jennifer Hadden is an Associate Professor in the Department of Political Science and at the Watson Institute for Public and International Affairs at Brown University. Her previous book with Cambridge University Press Networks in Contention: The Divisive Politics of Climate Change (2015) received four book awards from the American Political Science Association and the International Political Science Association.

• 1. Introduction; 2. A theory of INGO populations; 3. After the INGO explosion; 4. The growing specialization of INGOs; 5. The spreading out of INGOs; 6. The future of INGOs; 7. Appendix; Bibliography; Index.

'By integrating key concepts such as population density from organizational ecology to International Relations, Bush and Hadden offer a fresh perspective on the evolution of the non-profit sector both in the US and globally. Their meticulous research provides insightful answers to fundamental questions, such as why certain organizations lean toward hyper-specialization and under what conditions NGOs are more likely to compete with, rather than collaborate with, like-minded organizations in crucial areas of global governance.' Alexander Cooley, Barnard College, Columbia University