While international borders are important in understanding the “shape” of the international system and are part of those structural characteristics which affect the interaction opportunities of nations, little attention has been paid to their conceptualization, operationalization, and measurement. This paper undertakes four tasks to help fill this gap. The first is to indicate the potentially theoretical role that borders may play in international relations, discussing the relationships between distance/contiguity and interaction opportunities. The second task entails the conceptualization and measurement of international borders. The third task involves using the data derived from this framework to describe the international system in terms of borders for the period 1946‐1965. The fourth task is to indicate the utility of a border data set by addressing questions which have been posed in the international relations literature. Research results are presented for several questions concerning the relationships between borders and war, borders and alliances, and the diffusion of war.