Transgender and nonbinary young people experience some of the highest rates of violence of any group of young people. Violence is associated with a host of health concerns, including depression, anxiety, and suicidality. Gender minority stress theory describes increased risks for mental health concerns when experiencing violence targeting a person’s gender identity. Transgender and nonbinary young people experience high rates of child maltreatment, bullying, physical violence, sexual violence, and polyvictimization. Subsequently, transgender and nonbinary young people experience high rates of depression, anxiety, post-traumatic stress, homelessness, lack of access to healthcare, and lack of social support. While the research literature is clear that transgender and nonbinary young people experience these high rates of violence and poor health outcomes, there are little data on when these disparities emerge in the lives of transgender young people. Research shows that by the time transgender and nonbinary people reach adulthood they have experienced disparities and will likely continue to experience disparities in violence and health as transgender and nonbinary adults, but violence prevention efforts are stymied without a clearer understanding of the developmental timing of violence experienced by trans and nonbinary young people.Further, the extant literature on childhood and adolescent gender development suggests there are gendered differences in experiences of violence and health, but ongoing and historical approaches to child and adolescent development have perpetuated and reinforced scientific paradigms that explicitly frame transgender identity as pathological and non-normative, fundamentally excluding transgender and nonbinary children and adolescents from a scientific understanding of their development. This disregard has led to a dearth of data on the lived experiences of transgender and nonbinary children and adolescents, limiting any efforts toward improving their health and wellbeing. In order to address these gaps, this study asks the following questions: 1) How do transgender and nonbinary young people experience gender development during childhood, adolescence, and young adulthood? 2) What types of violence and sources of violence target transgender and nonbinary young people during childhood, adolescence, and young adulthood? 3) How does the social ecology, including social institutions and the agents of those institutions, interact with gender development or violence for transgender and nonbinary young people during childhood, adolescence, and young adulthood; and how do these social-ecological interactions contribute to risk or protective factors for violence, health, and safety?This qualitative study used a life history timeline (LHT) approach to collect lifecourse data on the experiences of violence and gender development during childhood, adolescence, and young adulthood with a geographically diverse sample of transgender and nonbinary young adults (N = 22; ages = 18-29 y/o). Qualitative, life history interviews were conducted on Zoom, using Zoom’s whiteboard function to construct the life history timeline. The transcripts were coded using an abductive constructivist grounded theory method to identify codes that emerged during the grounded theory coding process and a priori codes based on study aims and existing literature on gender identity and violence (Timmermans & Tavory, 2012). The domains included in the lifecourse interview were: landmarks (e.g., birthdays, academic milestones, prominent life transitions), gender development (e.g., gender awareness, gender identity, coming out, gender performance), violence (e.g., general polyvictimization domains), and social ecological features (e.g., cultural and political climate, family, peers, community features). A total of 22 transgender and nonbinary young people ages 18-29 were interviewed for the study. Abductive constructivist grounded theory analysis revealed key themes to answer the research questions.Participants described gender development occurring in three stages: childhood, adolescence, and young adulthood. The gender development that took place during these stages was different for trans binary participants compared with nonbinary participants. For the most part, trans binary participants understood and expressed their authentic gender during childhood, sometimes coming out explicitly to their families. Nonbinary participants did not feel a strong alignment with gender during childhood. During adolescence, trans binary participants took steps to more fully live in their authentic gender, even when those steps were to prepare for independence and adulthood. Nonbinary participants explored their gender during this time, which often coincided with exploring their sexual identity. During young adulthood, trans binary participants enacted steps to fully live in their authentic gender. Nonbinary participants identified with their authentic gender, usually after connecting with trans community and discovering the language of a nonbinary gender.Violence described by participants was primarily perpetrated by parents and other adult authority figures. Violence was predominantly motivated by gender role enforcement, in which perpetrators acted violently in order to correct, control, or punish gender performance that did not align with binary gender expectations. Anticipatory violence was described as violence that acted in anticipation, usually paired with gender role enforcement, of future forms of violence; such as enacting gender role enforcement in order to prevent future violence from bullies or other perpetrators who would act violently against gender role violations. A transmorphology of violence was revealed by participants, articulating a process by which the violence targeting transgender and nonbinary people changes over time in response to transitions on the part of young people (e.g., trans women experiencing increasing rates of misogynistic environmental sexual harassment as they transitioned more visibly into performing feminine gender). Social connections and affirmation were highlighted as the primary sources of information and safety for transgender and nonbinary youth during adolescence and young adulthood.This study articulates the gender development processes, experiences of violence, and social-environmental interactions for transgender and nonbinary young people during childhood, adolescence, and young adulthood. The refinement of the developmental timing of gender and violence along with the understanding of how gender role enforcement operates across their development provides further background for violence prevention efforts. Binary gender assumptions that are socialized onto young people from birth hold valence in how children are expected to behave, and enforcing sex-deterministic binary gender onto children, especially transgender children, can have enormous implications for their experiences of violence and mental, physical, and social health. By providing gender-affirming care and social support, we can improve safety and prevent future violence against transgender and nonbinary young people.