Systemic theory in international relations is an attempt to capture the relationship between the units of the international system (generally, the states) and the elements of the structure of the international system most relevant to their behavior. The goal is to capture the essence of international relations in the same simple and powerful manner that the heliocentric Copernican model captured the essence of astronomy. In practice, largely due to the complexity of the international system, systemic theory has been elusive in modern international relations. While systemic theorizing in international relations, in the form of balance-of-power theory, is centuries old, the theoretical complexities and empirical challenges of the scientific study of international systems are exceptionally daunting. Accordingly, very few attempts at a logically coherent, empirically supported systemic theory have been made, and far fewer are seen as unproblematic. Certainly no single model has achieved the degree of consensus that the Copernican model has in astronomy. At the same time, systemic theorizing is influenced to a much-larger degree than other forms of theorizing by systemic traditions in other disciplines—sociology, economics, and history, to name just a few. Because the origins, examples, and tests of systemic theories in international relations are so diverse and fragmented, this article will be broader than it is deep—an attempt to survey the landscape rather than to mine any one part of it comprehensively.