The seemingly natural areas of scholarly convergence between diplomatic historians and political scientists who focus on international relations have been underexploited, but there are also some signs that this may be changing. These include recent essays suggesting ways in which the two disciplines can contribute to each other; a number of prizewinning dissertations, later turned into books, by political scientists that effectively combine political science theories and historical materials; collaborative efforts among scholars in the two disciplines; interdisciplinary journals such as International Security that provide an outlet for historians and political scientists with common interests; and creation of a new section, “International History and Politics,” within the American Political Science Association. This book is an effort to contribute further to an exchange of ideas between the two disciplines by describing some of the theories, approaches, and “models” political scientists have used in their research on international relations during recent decades. A book cannot do justice to the entire range of theoretical approaches that may be found in the current literature, but perhaps those described here, when combined with citations of some representative works, will provide diplomatic historians with a useful, if sketchy, map showing some of the more prominent landmarks in a neighbouring discipline. Efforts of some political scientists to develop “formal” or mathematical approaches to international relations are neglected here; such abstract models are likely to be of limited interest to historians. The “post modern” challenge to all other theories and methodologies—the third “great debate”—will only briefly be described and evaluated. With these caveats, let me turn now to classical realism, the first of the systematic models to be discussed in this book. This textbook will be of indispensable help to students of this course both at the undergraduate and postgraduate level.