Animal studies have shown that mother-infant interactions can have long-term impacts on areas of the brain that regulate fearful behavior and the physiology of stress. Here, the research on human infants and children is reviewed with an eye to whether early experiences have similar effects in our species. Research shows that during the first year, sensitive and responsive caregiving becomes a powerful regulator of emotional behavior and neuroendocrine stress hormone activity in young children. Indeed, quality-of-care effects can be detected for children throughout the preschool years. Reviewed research suggests that temperament affects the likelihood that children will show increases in stress hormones as the quality of their care decreases. Finally, we review the literature on stress hormone activity in children who have been maltreated early in life, and explore the critical question of whether enhancing care later in development can reverse the effects on behavior and neurobiology of early adverse experiences.