Children growing up in the United States today typically spend a substantial portion of their early childhood years in early care and education (ECE) settings. These settings are thus an essential element of any effort to understand the ecology of early development. Research aimed at identifying the short- and long-term impacts of ECE experiences has a long history, the results of which now point to three key conclusions. (a) Although parents are the most important influence on children's development, ECE experiences have both short- and long-term impacts on a wide range of developmental outcomes that are best understood in interaction with family effects. (b) The quality of adult-child interactions in ECE settings is the most potent source of variation in child outcomes, although the amount of exposure to these settings also plays a role, perhaps especially with regard to social-emotional development. (c) Some children, notably those growing up in poverty, appear to be more vulnerable to variation in the quality of ECE settings than do other children. The frontiers of ECE research are addressing individual differences in children's responses to child care and approaching these settings both as sites for intervention research and as part of a wider web of important settings in young children's lives.