In any field of scholarly inquiry it is recognized that we must describe before we can hope toexplain. That is, we cannot account for the incidence of a certain class of events or conditions until we have identified and described those particular phenomena. If we agree that the current state of theory in the field of international organization leaves much to be desired, this fact may be partly due to our violation of this principle. Whether we deal with all international organizations over a lengthy period of time or a smaller subset based on such inclusion criteria as function or time period and whether we treat such organizations as the dependent, intervening, or independent variable, it is essential that we first acquire the data by which such organizations can be described. The major purpose of this article is to report the results of a first systematic effort to generate this data, so that we may move on in a cumulative fashion toward the empirical testing of propositions, models, or theories in which international organization is a major variable.