abstract: The transition from adolescence to young adulthood is an important developmental period, as youth experience rapid changes in many domains of their lives (Settersten & Ray, 2010). These transitions have been linked to both positive and negative turning points in youths’ behavior and psychosocial wellbeing (Elder & Shanahan, 2007). Being disengaged from work and school, two important social institutions involved in the transition to adulthood (Havighurst, 1972), has been associated with poorer mental health and increases substance use; in this literature, there is still a dearth of research among youth in the United States of America and on the developmental implications of disengagement (Hilley et al., 2019). Therefore, this dissertation includes two studies to address these gaps with respect to mental health and substance use. Study 1 explores the heterogeneity in youths’ engagement and disengagement from work and school at two developmentally relevant time points across the transition to adulthood rather than impose a priori definitions of disengagement. Next, this study explores whether risk and protective factors predict membership in these subgroups. Finally, this study examines subgroup differences in problematic substance use concurrently and longitudinally. Study 2 investigates the cross-lagged associations between opportunity youth (or youth who are neither in school nor working) status and mental health over the transition to adulthood and explores whether familial social support and socioeconomic status mitigate or exacerbate the influence of opportunity youth status on mental health. Findings from these studies support the developmental nature of disengagement (despite its heterogeneity) and its connection with mental health and substance use, as well as suggest the need for additional research into risk and protective factors. / Dissertation/Thesis / Doctoral Dissertation Family and Human Development 2020