Since Realism builds on fundamental insights about world politics and state action, progress in the study of international relations requires that we seek to build on this core. This chapter examines the argument of Thucydides and Morgenthau to extract the key assumptions of Classical Realism. It addresses the question of interpretation and puzzle-solving within the Realist tradition. The chapter considers the shortcomings of Realism when judged by the standards that Lakatos establishes, or even when evaluated by less rigorous criteria, and begin to ask whether a modified version of Structural Realism could correct some of these faults. The fruitfulness of contemporary Realist analysis is best evaluated by considering some of the finest work in the genre. Game theory has yielded some insights into issues of negotiations, crises, and limited war, most notably in the early works of Thomas Schelling. Realism, as developed through a long tradition dating from Thucydides, continues to provide the basis for valuable research in international relations.