Nationalism is a set of beliefs about the nation: its origins, nature, and value. For nationalists, we are particular social animals. On the one hand, our lives are structured by a profound sense of togetherness and similarity: We share languages and memories. On the other hand, our lives are characterized by deep divisions and differences: We draw borders and contest historical narratives. For nationalism, humanity is neither a single species-wide community nor an aggregation of individuals but divided into distinct and unique nations. At the heart of nationalism are claims about our identity and needs as social animals that form the basis of a series of normative claims. To answer the question “what should I do” or “how should I live,” one must first answer the questions “who am I” and “where do I belong.” Nationalism says that our membership in a nation takes precedence and ultimately must guide our choices and actions. In terms of guiding choice and action, nationalist thought proposes a specific form of partiality. Rather than treat the interests or claims of persons and groups impartially, the nationalist demands that one favors one’s own, either as a group or as individual persons. While nationalism does not claim to be the only form of partiality, it does claim to outrank all others: Loyalty or obligations to other groups or identities are subordinated to national loyalty. Together, these claims function as a political ideology. Nationalism identifies the nation as the central form of community and elevates it to the object of supreme loyalty. This fundamental concern for the nation and its flourishing can be fragmented into narrower aims or objectives: national autonomy, national identity, and national unity. Debate on nationalism tends to divide into two clusters, one descriptive and one normative, that only make partial contact. For historians and sociologists, the questions are explanatory: What is nationalism, what is a nation, how are they related, and when and how did they emerge? Philosophers and political theorists focus on the justification of nationalism or nationalist claims: Is national loyalty defensible, what are the limits of this loyalty, how do we rank our loyalties, and does nationalism conflict with human rights?