The Union of International Associations’ documentation on international non- governmental organizations (INGOs) is a treasure trove for historians and social scientists alike. INGOs are part and parcel of the modern world. They both reflect and influence social, political, cultural and economic conditions around the globe. They occupy a space of human activity in between, but connected to, the market and the state, and this space seems to be growing. The present chapter seeks to show how a historical investigation into this space might benefit from the use of UIA data. As other contributions to this volume have noted, the UIA aims to register the key activities of all INGOs that exist or have ever existed. As anyone familiar with historical databases would immediately admit, a database with such a large geographical (the globe) and temporal span (in principle that of human history, though in practice mostly the past two centuries) cannot, and never will be, complete in its coverage. This is also acknowledged by the UIA and is partly caused by the fact that most of the data is provided by the organizations themselves. For the present purpose – namely the visualization of various long-term developments – we will, however, assume that it can nonetheless offer a representative picture. For practical reasons, we focus on the period between 1800 and 1970, concentrating on two UIA datasets: the annual congress calendar and the Yearbook series, featuring all organizations that meet the UIA’s definition of an INGO.