From the mid-nineteenth century, Britain experienced a 'detective craze'Â in popular culture. The girl detective emerged comparatively late in the first decade of the twentieth century. Despite the prominence and permeation of the girl detective in British experiences of girlhood, the cultural and social meanings of this character have not been significantly explored by historians of detective fiction and childhood. In particular, the proliferation of the girl detective ---Â a profession symbolising the status quo and the rule of law --- as women's opportunities within the nation were expanding has not been considered in histories of gender, youth, and citizenship. This dissertation analyses the girl detective as both a cultural product and embodied role for girls at different historical moments in the first half of the twentieth century. It examines representations of the girl detective in Girl Guide training, girls'Â story papers, Warner Bros.' Nancy Drew film series, and Enid Blyton's child detective novels and fan magazine. This thesis argues that the girl detective communicated a model of idealised girlhood and female citizenship consistent with the conservative modernity characteristic of British society in the early-twentieth century. In particular, the girl detective emphasised girlsÃÂ¢ÃÂÃÂ detective work as moralistic and pertaining to the identification of need, engaging with contemporary understandings of middle-class public womanhood and philanthropy while also providing girls with the opportunity to traverse boundaries of age, gender, and class imaginatively and in real life. This study contributes to existing debates on the nature of mass culture, girlhood, and citizenship in Britain, examining the dissemination of models of conservative modernity in girl culture and the extent to which youthful female citizenship reinforced traditional ideas of femininity, with particular focus on affect and emotion.