Using data from the NICHD Study of Early Child Care (n = 1364), we examined how experiences in early childcare (ECE) and in families from 6 to 54 months predicted the development of first-grade prosocial behaviour (PSB), a core component of social and emotional success. SEM analyses revealed several key pathways: (1) both ECE process quality and structural quality predicted later PSB, and the indirect path to PSB from ECE structural quality through process quality was significant; (2) indirect paths to PSB from family demographic factors (minority; income/education) through ECE process quality were also significant; (3) dynamic family processes predicted PSB directly, and indirect paths to PSB from family structure and demographics through family processes were significant; (4) ECE process quality predicted family dynamic qualities and the indirect path from ECE process quality to PSB through family dynamic processes was significant. Developmental and policy implications are discussed.