Other papers in this special edition provide evidence to implicate activity of the limbic hypothalamic-pituitary-adrenocortical (L-HPA) system in the etiology of drug and alcohol abuse. Furthermore, studies in rodents and primates suggest that responsivity and regulation of this system later in life may be shaped by social experiences during early development. Cortisol is the major hormonal product of the L-HPA system in humans. Although it provides only a partial understanding of the activity of this neuroendocrine axis, its regulation may bear importantly on human growth and development. We review developmental studies of cortisol and behavior in human children, birth to approximately 5 years of age. We describe the development of social buffering of cortisol responses that produces a functional analogue of the rodent stress hyporesponsive period by the time children are about 12 months of age. We further describe the sensitivity of cortisol activity to variations in care quality among infants and toddlers, along with evidence that children with negative emotional temperaments may be most likely to exhibit elevations in cortisol under conditions of less than optimal care. Finally, the few studies of cortisol activity under conditions of neglectful and abusive care of young children are considered, noting that these often have yielded evidence of reduced rather than increased cortisol levels.