Nothing was more important for eugenicists than intelligence. A means to measure intelligence and identify feeble-mindedness was central to the policies that eugenics promoted. ‘Eugenic intelligence’ explains the definition and classification of feeble-mindedness that resulted in the segregation and incarceration of thousands of people. It outlines the origins of intelligence testing with French psychologists Alfred Binet and Théodore Simon in 1905 and the mass application of intelligence testing that informed the social policies of the early twentieth century. It also explores the belief that feeble-mindedness was the actual cause of undesirable social behaviors; explains the links between race and intelligence; and discusses the critics who rejected the principles behind intelligence testing and disavowed a purely hereditary understanding of intelligence.