Project Summary/Abstract This R01 application responds to the National Institutes of Health Notice of Special Interest in Research on the Health of Sexual and Gender Minority (SGM) Populations (NOT-MD-19-001). Sexual minority youth (SMY) face a number of mental health disparities, including high rates of depression and suicidality. Sexual minority identity development includes several key milestones (e.g., becoming aware of non-heterosexual attractions, questioning one’s sexual orientation, self-identifying as a sexual minority, and coming out to others) that typically occur during adolescence. Challenging experiences that occur during the sexual identity development process related to social stigma and stress likely contribute to mental health problems for this population; however, this area has been understudied and is poorly understood. To address critical gaps and limitations in this area, this project aims to: (1) Identify common patterns and heterogeneity in sexual identity development milestones among contemporary SMY; (2) Determine the extent to which sexual identity development relates to mental health problems (i.e., depression and suicidality) among SMY; and (3) Identify coping strategies and resources used by SMY to navigate stressful milestone experiences. To accomplish these aims, we will recruit a diverse sample of 315 SMY who will complete a one-time interview that will include life history calendars, which is an innovative method used to collect retrospective life-course data. Life history calendars will capture mixed-methods data about the sexual minority identity development process and mental health problems during adolescence. Analyses will include descriptive statistics, latent profile analysis, regression modeling, qualitative content analysis, and data visualization techniques. This project will result in (1) a rich and nuanced understanding of a complex and dynamic developmental process—sexual identity development among contemporary SMY, (2) identification of which milestones involve distressing experiences for SMY, (3) understanding why certain milestones are distressing and how these events may be associated with depression and suicidality, and (4) a framework of coping strategies and resources used by SMY to cope with stress during identity development.