Over one out of every five children in the United States lives in poverty. Worldwide, the figure is one out of every two children. Ten million children die each year, most of them in impoverished countries in sub-Saharan Africa and South Asia. The experience of growing up in poverty appears to have both short- and long-term negative consequences. Poor children have higher rates of acute and chronic diseases, and may have worse physical and mental health in adulthood. What we are currently seeking to understand is how socioeconomic status affects health. Even after taking into account factors such as medical care, diet and nutrition, social support, and health behavior, studies of health outcomes still generally find a large effect attributable to socioeconomic status. What is it about social class or social stratification in and of itself that is important for health, both during childhood and in adulthood?