Adolescence is a highly sensitive life period, during which social relationships with peers may have powerful repercussions for youth development, some of which can last a lifetime. With the transition to adolescence, youth become increasingly sensitive to their peers, and they develop a strong motivation to form and maintain positive relationships with them. Adolescents who struggle to achieve this goal (e.g., friendless) and those who are exposed to peer adversity (e.g., victimization, rejection), have been shown to be at increased risk for both mental and physical health problems. Research has begun to reveal that such effects may be visible even several years after exposure, when youth become adults. Yet, the mechanisms underlying the long-lasting effects of peer adversity remain largely unknown. This project aims to address this research gap in two ways. First, we will examine how peer adversity may modify adolescents’ daily life across multiple domains of functioning, thus posing risks for their well-being. For this, a sample of adolescents will be followed through the first years of secondary school using a measurement-burst design, with repeated assessments in daily life. Specifically, the experience sampling methodology (ESM) will be used to assess adolescents’ emotional, cognitive and physiological functioning in real-time in their real-world. Notably, the ESM will incorporate assessments of multiple physiological processes, including electrodermal activity (EDA) and heart rate variability (HRV), measured via wearable sensors. Monitoring these processes continuously in real-life will allow us to examine how peer adversity may shape adolescents’ functioning by modifying micro-level within-person dynamics between social, emotional and physiological responses. To analyze these intensive longitudinal data, the newly developed analytic framework Dynamic Structural Equation Modeling (DSEM) will be employed. Second, biomarkers of immune system activity (e.g., Interleukin 6) will be assayed in previously collected data, to examine whether changes in inflammatory markers mediate the link between peer adversity and adolescent health. Altogether this project will yield unique data at multiple level of analyses which may contribute to shed light on the pathways through which peer adversity affect adolescent health and development.