Unlike the theory of international trade and payments, theorizing on international relations or the international “system” is of fairly recent origin, has attracted relatively few scholars, and—in terms of achievement—is still in a rather underdeveloped stage. But there has in fact been important progress during the last twenty-five years, much of it along tentative, experimental, and not necessarily congruent lines. Nowadays there seems to be new interest in such theorizing and the Center of International Studies at Princeton University thought it useful to collect a reasonably representative sample of recent thinking and, toward this end, to call upon younger scholars in the field rather than on the established leaders, such as Harold Lasswell, Hans Morgenthau, Richard Snyder, Arnold Wolfers, and Quincy Wright.