This chapter examines the key ideas and concepts of nationalism as ideology. It first defines nationalism and considers how the nation is socially constructed as an imagined community. It then analyses the practical implications of nationalist ideology in terms of the functioning of the nation-state (and of nationalist political parties). It also looks at the ‘rational’ form of nationalism (that is, the civic variety) and its ‘sticky’ connections to liberalism and socialism; the link between nationalism and politics; and the relationship between nationalism and globalization. The rational and somewhat pragmatic nationalism is compared with the ‘irrational’ and emotional variant found within both conservatism and fascism. The chapter concludes by highlighting key lessons regarding nationalism as ideology. Case studies relating to Scottish national identity, Brexit, Chinese nationalism, and ethnic nationalism in Russia are presented.