At the heart of nationalism lies the belief that humanity is (or ought to be) divided into nations, and that nations are (or ought to be) the basis of independent sovereign states. The basic problem of nationalism is the difficulty (if not impossibility) in making political facts correspond to the national ideal. As a result, scholarship on nationalism deals with complex issues regarding the nature of identity and belonging, as well as the origins and legitimacy of the international system of nation-states. Consequently, this article has not been designed to answer questions, but rather to introduce the international-relations reader to key debates, developments, and controversies in a field that is deeply interdisciplinary. The article comprises works from disciplines as varied as political science, sociology, anthropology, law, and criminology as well as international relations. It has been split into three main sections. The first deals with nationalism theory, which is mostly concerned with the origins of nations and their relationship to the state. The second section takes the study of nationalism further by looking at identity, national or otherwise, and its relationship to belonging. Finally, as long as nationalism has existed, there has been conflict in its name. The last section thus looks at instability and nationalism in its myriad of forms, including secession, self-determination, multiculturalism, and ethnic conflict.