The years immediately preceding the First World War witnessed the development of a significant body of literature claiming to establish a ‘science of internationalism’. This article draws attention to the importance of this literature, especially in relation to understanding the roles of non-governmental organizations in world politics. It elaborates the ways in which this literature sheds light on issues that have become central to 21st-century debates, including the characteristics, influence and legitimacy of non-governmental organizations in international relations. Among the principal authors discussed in the article are Paul Otlet, Henri La Fontaine and Alfred Fried, whose role in the development of international theory has previously received insufficient attention. The article concludes with an evaluation of potential lessons to be drawn from the experience of the early 20th-century ‘science of internationalism’.