This book sets out a measure of authority for seventy-six international organizations (IOs) from 1950, or the time of their establishment, to 2010 which can allow researchers to test expectations about the character, sources, and consequences of international governance. The international organizations considered are regional (e.g. the EU, Andean Community, NAFTA), cross-regional (e.g. Commonwealth of Nations, the Organization of Islamic Cooperation), and global (e.g. the UN, World Bank, WTO). Firstly, the book introduces carefully constructed estimates for the scope and depth of authority exercised by international governments. The estimates are unique in their comparative scope, their specificity, and time span. Secondly, it describes describe broad trends in IO authority by comparing delegation and pooling, over time, across IOs, and across decision areas. Thirdly, it presents the evidence gathered by the authors to estimate international authority by carefully discussing forty-seven international organizations, and showing how their bodies are composed, what decisions each body makes, and how they make decisions.