This text argues that theory is central to explaining International Relations (IR) and that the discipline of IR is much more relevant to the world of international relations than it has been at any point in its history. Some chapters cover distinct IR theories ranging from realism/structural realism to liberalism/neoliberalism, the English school, constructivism, Marxism, critical theory, feminism, poststructuralism, green theory, and postcolonialism. Oher chapters explore International Relations theory and its relationship to social science, normative theory, globalization, and the discipline’s identity. This introduction explains why this edition has chosen to cover these theories, reflects on international theory and its relationship to the world, and considers the kind of assumptions about theory that underlie each of the approaches.