This report summarizes findings from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development as effect sizes for exclusive maternal care and--for children in child care--type, quality, and quantity of care. Children (n = 1,261) were recruited at birth and assessed at 15, 24, 36, and 54 months. Exclusive maternal care did not predict child outcomes, but multiple features of child-care experience were modestly to moderately predictive. Higher quality child care was related to advanced cognitive, language, and preacademic outcomes at every age and better socioemotional and peer outcomes at some ages. More child-care hours predicted more behavior problems and conflict, according to care providers. More center-care time was related to higher cognitive and language scores and more problem and fewer prosocial behaviors, according to care providers. Child-care effect sizes are discussed from 3 perspectives: (a) absolute effect sizes, reflecting established guidelines; (b) relative effect sizes, comparing child-care and parenting effects; and (c) possible individual and collective implications for the large numbers of children experiencing child care.