This book sets out a measure of authority for seventy-six major international organizations (IOs) from 1950 to 2010 in an effort to provide systematic comparative information on international governance. On the premise that transparency is key in the production of data, the authors chart a path in laying out the assumptions that underpin the measure. Successive chapters detail the authors’ theoretical, conceptual, and coding decisions. In order to assess their authority, the authors model the composition of IO bodies, their roles in decision making, the bindingness of IO decisions, and the mechanisms through which they seek to settle disputes. Profiles of regional, cross-regional, and global IOs explain how they are composed and how they make decisions. A distinctive feature of the measure is that it breaks down the concept of international authority into discrete dimensions. The Measure of International Authority (MIA) is built up from coherent ingredients—the composition and role of individual IO bodies at each stage in policy making, constitutional reform, the budget, financial compliance, membership accession, and the suspension of members. These observations can be assembled—like Lego blocks—in diverse ways for diverse purposes. This produces a flexible tool for investigating international governance and testing theory.