The study of international relations is a subfield of political science that focuses on the ways that different states interact with one another. The main goal of all relationships among states is to work toward making the international system one of peace—friendly, nonviolent relations among states—and balance—an equal distribution of the chance for any country to take the lead in setting the course for the international system. As with all things related to politics, who has the means to exert their power to guide the action of members of the political community determines who gets to set the terms of “peace” and be in charge of what “balance” looks like. Power can best be defined as the ability to establish and enforce the rules to which all other actors in a system must adhere. This chapter looks at the different players in the international system and how they interact. It examines the principles that guide the establishment of the political, social, and economic environment in which these interactions take place.