Project Summary/Abstract Sex represents a critical variable not only in typical neurodevelopment, but also neuropsychiatric disease. Substantial sex differences have been identified in prevalence, course and outcome of psychiatric disorders, with prior evidence indicating this occurs in a time-dependent fashion throughout the lifespan, with specific epochs in early development and in the pubertal period. Despite clear epidemiological evidence for these disparities, understanding of underlying biological mechanisms in humans is limited. Animal studies suggest a critical role of sex hormones in both developmental periods, however translation of these mechanisms to humans has been hampered by the lack of an experimental model that allows segregation of effects related to early organizational effects and later activational processes in puberty. The recent advent of cross-sex hormone therapy (CSHT) in adolescents with gender dysphoria, presents a unique opportunity to investigate these processes, while also advancing the empirical basis of clinical care for this vulnerable population of youth. While early data from comparable European clinics demonstrated that adolescents in late puberty receiving GnRH treatment showed improved psychological functioning and unanimously opted to proceed with CSHT in late adolescence, there is little to no empirical data guiding clinical practices of CSHT in early pubertal adolescents, highlighting the need for further research to address the critical knowledge gap in this cohort of youth. The research we propose has never been conducted in early pubertal adolescents, an important point given the prevailing theoretical framework which posits that hormonal effects are exerted in defined, time- sensitive developmental windows, with early and permanent organizational effects in the perinatal period, and further activational changes occurring during adolescence. Studying an adolescent cohort is critical for this population as it (1) establishes a neural basis for sex steroid influence in puberty as manifested in brain anatomy and connectivity through a model of gender dysphoria and cross-sex hormone therapy, (2) identifies independent processes by which estrogens and testosterone differentially affect individuals with a natal sex background of either female- or male-typical early organizational effects and (3) maps these findings onto behaviors that are most strongly associated with sexually dimorphic patterns of psychopathology in the adolescent period. Results from this interdisciplinary proposal will provide a much-needed foundation for understanding the longitudinal impact of treatments that are already being used in clinical settings and allow better understanding of risk factors associated with disease that emerges along sex-specific trajectories from the pubertal period, greatly improving clinical outcomes for both typically developing and GD youth.