This Monograph presents the results of the Early Intervention Collaborative Study, a longitudinal investigation of the cognitive and adaptive behavior development of children with developmental disabilities and the adaptation of their parents, extending from infancy through middle childhood. The study was designed to generate and test conceptual models of child and family development and contribute to the knowledge base that informs social policy and practice. The sample for the investigation reported here consists of 183 children with Down syndrome, motor impairment, developmental delay and their families who were recruited at the time of their enrollment in an early intervention program in Massachusetts or New Hampshire. Data were collected at five time points between entry to early intervention and the child's 10th birthday. Home visits were conducted at each time point and included child assessments, maternal interview, and questionnaires completed independently by both parents. Trajectories in children's development and parental well-being were analyzed using hierarchical linear modeling. Predictor variables were measured at age 3 years when children were exiting early intervention programs. Children's type of disability predicted trajectories of development in cognition, social skills, and daily living skills. Children's type of disability also predicted changes in maternal (but not paternal) child-related and parent-related stress. Beyond type of disability, child self-regulatory processes (notably behavior problems and mastery motivation) and one aspect of the family climate (notably mother-child interaction) were key predictors of change in both child outcomes and parent well-being. A different aspect of the family climate--family relations--also predicted change in child social skills. Parent assets, measured as social support and problem-focused coping, predicted change in maternal and paternal parent-related stress respectively. The implications of these findings for both the science of child development and the policies and practices of developmental intervention are discussed.