The past half-century has seen a dramatic rise in both the number of intergovernmental organizations (IGOs) and participation levels by states and other actors. This exponential growth has been tracked increasingly through the collection and analysis of quantitative data. Some data collections can be divided according to the major subfi elds of international relations (security, international political economy), while others focus on key institutions: the United Nations (UN), the European Union (EU), the International Monetary Fund (IMF), and the World Trade Organization (WTO). Much less attention has been devoted to collecting data on minor IGOs, or to specialty organizations, which are no doubt worthy of additional scrutiny. In addition, very few large data collection efforts exist for regional organizations (with the notable exception of the EU), or for international non-governmental organizations.