Abstract: From the turn-of-the-century inception of the concept of modern adolescence, experts who articulated it had to grapple with questions of homosexuality and gender diversity. The construction of adolescence as a life period most vulnerable to 'tendencies' that would become fixed in maturity compelled experts to manage, diagnose, and treat youth for same-sex desires and activities. As adolescence became a standardized period in the developmental life cycle, the emerging modern sexological and psychological concepts of homosexuality and heterosexuality similarly incorporated it into theories of origin and adjustment. By the 1930s, understandings of modern adolescence and modern homosexuality had become mutually reliant upon one another. This had profound effects on the experts, institutions, and youth cultures shaping adolescence and broader society. This project explores the crucial moment when adolescence and homosexuality became disciplinary, productive, and cultural forces in the expert discourses, institutions, and lives of Americans. One of the most perilous threats to development in psychologist G. Stanley Hall's turn-of-the-century promotion of modern adolescence came in the maladjustments of gender confusion or sexual misdirection. These could emerge in the 'lag' between the onset of sexual desires and the establishment of necessary social and moral sublimations. Developmental psychology in the 1910s and 1920s accommodated an acceptable homosexual 'phase' and encouraged only gentle interventions in 'normal,' primarily white and middle-class, adolescence. For the mostly working-class and nonwhite youth marked as delinquent, however, homosexuality and gender transgression were often taken as signs of deeper mental disorder or criminality, and justified severe forms of treatment. Chicago School sociologists democratized modern adolescence through rehabilitating some aspects of working-class youth culture. Sociologists saw the gang in terms of its potential to develop boys toward mature masculinity and heterosexuality, and they refused evidence of less heteronormative aspects of gang life. Urban spaces intersecting youth and homosexuality in early twentieth-century Chicago reveal diverse identities, opportunities, pleasures, and concepts of belonging that exceeded expert models of adolescence. Yet because girls and boys could not fully escape the discourses and institutions of modern adolescence, these spaces involved a negotiation between experts, institutions, and young people that continues today.