PROJECT SUMMARY/ABSTRACT The proposed research would capitalize on a wealth of longitudinal data previously collected at key developmental stages over a period of 27 years as part of the Harlem Longitudinal Development Study. It has two major aims: The first is to identify the mediators of the relationships between social determinants of health (i.e., socioeconomic disadvantage and ethnicity-based/racial discrimination) and allostatic load in a sample of African Americans and Puerto Ricans entering midlife. The research proposal is guided by a contextual- developmental model based in Phenomenological Variant of Ecological Systems Theory, as well as a pathophysiological theory of stress (allostatic load). Second, the research will identify the differential developmental trajectories of perceived ethnicity-based/racial discrimination from late adolescence to the early forties, examine their associations with allostatic load and mental health in the early forties, and test the ability of resilience resources to mitigate these associations. The sample for this study will consist of 400 male and female African Americans and Puerto Ricans who were previously seen in adolescence (T1, T2), emerging adulthood (T3), young adulthood (T4), and adulthood (T5, T6). Identifying the pathways through which risk and protective factors for allostatic load operate will have important implications for public health by providing targets for intervention and informing policy. Finding moderating constructs that can mitigate the deleterious sequelae of chronic ethnicity-based/racial discrimination will highlight significant areas of resilience. Such a focus on resilience not only moves away from a deficit perspective of people of color, but also has the potential to transform policy and interventions by building on strengths, rather than treating deficits. A move in this direction is an important step towards reaching the goal of eliminating the significant social and ethnic/racial health disparities that plague the U.S. and realizing health equity for all.