This article reformulates liberal international relations (IR) theory in a nonideological and nonutopian form appropriate to empirical social science. Liberal IR theory elaborates the insight that state-society relations—the relationship of states to the domestic and transnational social context in which they are embedded—have a fundamental impact on state behavior in world politics. Societal ideas, interests, and institutions influence state behavior by shaping state preferences, that is, the fundamental social purposes underlying the strategic calculations of governments. For liberals, the configuration of state preferences matters most in world politics—not, as realists argue, the configuration of capabilities and not, as institutionalists (that is, functional regime theorists)maintain, the configuration of information and institutions. This article codifies this basic liberal insight in the form of three core theoretical assumptions, derives from them three variants of liberal theory, and demonstrates that the existence of a coherent liberal theory has significant theoretical, methodological, and empirical implications. Restated in this way, liberal theory deserves to be treated as a paradigmatic alternative empirically coequal with and analytically more fundamental than the two dominant theories in contemporary IR scholarship: realism and institutionalism.