Transgender and gender expansive youth (TGEY), including adolescents and emerging adults (i.e. ages 16-29) experience disproportionately negative mental health outcomes in the United States. TGEY of color carry the greatest burden of outcomes, experiencing the highest rates of suicide, depression, anxiety, and trauma-related stress disorders when compared to their non-Hispanic white counterparts. Literature indicates stigma as a significant driver of these disparities. TGEY of color experience heightened and altogether unique experiences of stigma associated with their multiple marginalized identities (i.e. racial/ethnic identity, gender identity, and sexual identity) that impact their mental health. To examine these experiences in a scientifically rigorous and authentic manger, it is imperative that these experiences be captured in a way that maximizes content and ecological validity. There is, however, a lack of psychometrically validated measures of intersectional stigma that are informed by the experiences of TGEY of color. This reflects a limitation of using strictly quantitative methods for understanding intersectional experiences. A true intersectional approach to measuring and understanding stigma experiences of TGEY of color requires an events-level approach that can capture the contextual aspects of intersectional stigma experiences, as well as the qualitative nuances that help to fully flesh out any subtle, contextual elements of these stigma experiences. The goal of this dissertation study is to conduct a two-part study using methodology and measures developed for the PRISM project (PRISM; R21-MH121311; PI: Rendina). The original PRISM measure aimed to capture event-level experiences of intersectional stigma among cisgender young sexual minority men (YSMM) utilizing an ongoing nationwide cohort study of 150 YSMM who were at risk for HIV. Study 1 of this dissertation project will aim to iteratively enhance the item pool of the PRISM measure for utility among TGEY of color. For this study, we will conduct cognitive interviewing to gather feedback from 10 TGEY of color to ensure the item bank represents the range of daily stigma experiences of TGEY. Study 2 will aim to test the psychometric validity of the PRISM measure among TGEY of color. Here, we will establish reliability and validity of the PRISM measure and test the concurrent validity of its items with mental health outcomes among our sample of TGEY. Study 2 will also include a qualitative component that aims to contextualize quantitative findings. These interviews will be conducted among 15 randomly selected participants and will contextualize the experiences of intersectional stigma and mental health impacts that were quantitatively captured in Aim #2. The research and training plans guiding this proposal have been carefully crafted to optimize my ongoing doctoral training experiences and foster my development as a future independent research scientist.