This Monograph presents the results of a nonexperimental, longitudinal investigation of developmental change in 190 infants and their families after 1 year of early intervention services. The Early Intervention Collaborative Study (EICS), conducted in association with 29 community-based programs in Massachusetts and New Hampshire, was designed to assess correlates of adaptation in young children with disabilities and their families over time, to inform social policy by analyzing the influences of family ecology and formal services on child and family outcomes, and to generate conceptual models to guide further investigation. The study sample (mean age at entry = 10.6 months) includes 54 children with Down syndrome, 77 with motor impairment, and 59 with developmental delays of uncertain etiology. Data were collected during two home visits (within 6 weeks of program entry and 12 months later) and included formal child assessments, observations of mother-child interaction, maternal interviews, and questionnaires completed independently by both parents as well as monthly service data collected from service providers. Child and family functioning varied considerably. Developmental change in the children (psychomotor abilities, adaptive behavior, spontaneous play, and child-mother interaction skills) was influenced to some extent by gestational age and health characteristics, but the strongest predictor of change was the relative severity of the child's psychomotor impairment at study entry. Families demonstrated generally positive and stable adaptation (in terms of the effect of rearing a child with disabilities on the family, parenting stress, and social support), despite persistent challenges with respect to mother-child interaction and differences in reported stress between mothers and fathers. Documentation of services revealed that early intervention is a complex and multidimensional experience that spans multiple public and private systems. Vulnerable and resilient subgroups within the sample were identified, and different correlates of adaptive change were demonstrated. Results of data analyses suggest new perspectives on the study of early childhood disability. The implications of the findings for developmental theory and social policy are discussed.