This book combines transnational history with the comparative analysis of racial formation and reproductive sexuality in the settler colonial spaces of the United States and British Australia. Specifically, the book places whiteness and the changing definition of what it meant to be white in nineteenth-century America and Australia, at the center of our historical understanding of racial and sexual identities. In both the United States and Australia, whiteness was defined in opposition to the imagined cultural and biological inferiority of the Indian, Negro, and Aboriginal savage. Moreover, Euro-Americans and Euro-Australians shared a common belief that whiteness was synonymous with the extension of settler colonial civilization. Despite this, two very different understandings of whiteness emerged in the nineteenth century. The book therefore asks why these different racial understandings of whiteness – and the quest to create culturally and racially homogeneous settler civilizations – developed in the United States and Australia.