This chapter outlines the basic theoretical approaches that are the foundations of international relations and are critical to understanding the field. As a start- ing point, we will begin with realist/power politics, as articulated by Hans J. Morgenthau. This has been one of the founding tenets of international relations since the end of World War II. (His seminal text, Politics Among Nations, was initially published in 1948.) Since then, the international political landscape has changed; new organizations tied to the notion of collective security assumed idealistically that security could best be assured not by having nations increase their power but by working cooperatively toward common goals and ends that would benefit all. Thus, a competing or (perhaps more appropriately) alterna- tive theory of international relations was born, which challenged the basic prin- ciples of realism. This new approach focused more on cooperation between and among nations rather than competition for power, and it embodied many of the ideals earlier espoused by Woodrow Wilson. Referred to as liberal theory, it incorporates economic ideas as well as political ones, and it has grown in prominence and importance since the end of the Cold War. Hence, the changes in the international system have contributed to a proliferation of other theories, all of which were designed to explain on a macro level, or more often on a micro level, some aspect of international relations.