This is the first book-length study of the ideological foundations of British imperialism in the twentieth century. Drawing on the thinking of imperial activists, publicists, ideologues, and travelers such as Lionel Curtis, John Buchan, Arnold White, Richard Jebb and Thomas Sedgwick, this book offers a comparative history of how the idea of imperial citizenship took hold in early twentieth-century Britain, and how it helped foster the articulation of a broader British world. It reveals how imperial citizenship as a form of imperial identity was challenged by voices in both Britain and the empire, and how it influenced later imperial developments such as the immigration to Britain of 'imperial citizens' from the colonies after the Second World War. A work of political, intellectual and cultural history, the book re-incorporates the histories of the settlement colonies into imperial history, and suggests the importance of comparative history in understanding the imperial endeavour. It will be of interest to students of imperialism, British political and intellectual history, and of the various former dominions.