Introduction While nations and nationalism have become the dominant mode of ascribing political culture in world politics, understanding the meaning and political importance of these terms has been a notoriously challenging task. One survey of concepts in International Relations (IR) said of the term ‘ national interest ’ that it was ‘the most vague and therefore easily used and abused’; of nationalism, that ‘there is a lack of consensus about what it is and why it has maintained such a firm hold over so much of the world's population’; and that ‘Nations and states seem identical but they are not’ (Griffiths and O'Callaghan 2002: 202–13). The discussion that follows will survey debates on nation and nationalism around four broad questions. The first concerns debates around terminology and their contemporary relevance for the study of IR. The second relates to questions of nation formation and the origins of nationalism, particularly in terms of how it came to shape modern states and international society . The third explores how the ideas of nations and nationalism have been important in IR theory and practice. The fourth section briefly considers the rise of new nationalism in contemporary world politics. As a discipline , IR has made a surprisingly modest contribution to this scholarship (see Carr 1945; Hinsley 1973; Mayall 1990; Breuilly 2013). Nationalism is often not addressed explicitly, but it has a significant tacit presence in all of the major schools of thought in the discipline. Consequently, mainstream IR theories have compounded some of the analytical problems associated with understanding nationalism. For example, classical realists have tended to conflate nation and state into the concept of ‘national interest’, while liberal and Marxist theorists have been internally conflicted over the merits of nationalism versus its potential to undermine ideals of internationalism. The study of nationalism should be a central consideration for any analysis of the major issues in contemporary global politics because taking questions of national interest, values and identity seriously is one way of invoking the idea that culture and ‘people’ matter.