Project Summary Young men who have sex with men (YMSM) between the ages of 18-29 are disproportionately affected by HIV, STIs, and poor mental health. The use of dating apps to find sexual or romantic partners among YMSM is nearly ubiquitous and has been associated with HIV and STI risk behaviors (e.g., condomless anal sex; CAS), though research on this association has been mixed. Recently, cross-sectional research and anecdotes from mental health providers have suggested that dating app use may be associated with poorer mental health and self-esteem among YMSM. Poorer mental health, in turn, has been linked to CAS and other HIV risk behaviors in this population. A small number of daily diary studies have linked nonclinical affective states (e.g., anxiety, negative arousal) with engagement in CAS. Results of these studies are mixed, though all suggest some prospective association between affect and HIV risk. However, none of these daily diary studies examined HIV risk in the context of dating app use because they collected data before dating apps became widespread. Thus, it may be that affective consequences of dating app use constitute an understudied mechanistic link between dating app use and engagement in HIV risk behavior. Given that no research to date has examined associations between dating app use and affect among YMSM longitudinally, the proposed daily diary study’s first aim is to examine prospective associations between daily dating app use and affective states among YMSM. Its second aim is to test the mediating effect of negative affect on the association between dating app use and CAS. Additionally, none of the aforementioned daily diary studies examined the effect of pre-exposure prophylaxis (PrEP; a medication that is nearly perfect at preventing HIV seroconversion when taken as directed) use on the link between affect and CAS because data were collected prior to the advent of PrEP. This constitutes a major gap in our understanding of HIV risk among YMSM because PrEP has been shown to affect their sexual risk appraisal and emotional associations with sexual partnering. Thus, to complement the Specific Aims described above, I will conduct exploratory analyses (e.g., sensitivity and moderation analyses) to better understand the impact of PrEP use on dating app use, negative affect, and sexual behavior among YMSM. The proposed study will allow me to build upon my training in qualitative and basic statistical methods by providing training in intensive longitudinal data analysis (i.e., multilevel modeling, mediation analyses, and moderation analyses). It will also afford me the opportunity to build upon my skills in recruitment, retention, and related longitudinal methods. Specifically, I will use resources and infrastructure available to me at Northwestern University and the Institute for Sexual and Gender Minority Health and Wellbeing to recruit a nationwide sample of YMSM that is diverse in terms of urban/rural locality and racial/e...