This book explores the international diffusion of Participatory Budgeting (PB), a local policy created in 1989 in Porto Alegre, Brazil, which has now spread worldwide. The book argues that the action of a group of individuals called "Ambassadors of Participation" was crucial to make PB part of the international agenda. This international dimension has been largely overlooked in the vast literature produced on participatory democracy devices. The book combines public policy analysis and the study of international relations, and makes a broad comparative study of PB, including cases from Latin America, Europe, and Sub-Saharan Africa. The book also presents a new methodology developed to examine PB diffusion, the "transnational political ethnography", which combines in-depth interviews, participant observation and document analysis both at the local and transnational level.