The Young Adolescent Project (YAP) is an ongoing longitudinal study investigating the effects of abuse and neglect on adolescent development. It is a multidisciplinary study guided by a developmental, ecological perspective, and designed to consider the physical, social, and psychological effects of childhood maltreatment through the transition from childhood to adolescence. Four waves of data collection have been completed, ranging from early (Mean age = 10.95) to late adolescence (Mean age = 18.24). Members of the maltreated group (n = 303) were selected from new cases that had been opened by the Department of Child and Family Services, whereas the comparison group (n = 151) were not involved with child welfare but lived in the same neighborhoods as the maltreated group. The study assessed a wide variety of domains including physical development (e.g., height, weight, body mass, pubertal development); physiological reactivity (e.g., cortisol); cognitive abilities; mental health (e.g., symptoms of depression, anxiety, trauma, and aggression); risk behavior (e.g., sexual activity, delinquency, or substance use); social development (e.g., self-esteem, competence, and social support); family environment; and exposure to community violence. Overall, our findings demonstrated the pervasive and persistent adverse effects of child maltreatment both within and across domains, but they also identified maltreated youth with positive functioning. Our hope is that this work will help move us toward identifying targets for intervention to cultivate resilience and positive adaptation after early maltreatment experiences.