Mortality rates in the developed world have fallen sharply during the twentieth century. Individuals of lower socioeconomic status, however, generally have faced higher mortality rates than individuals of higher status. The literature documenting the relationship between socioeconomic status and health is reviewed, including several recent contributions and evidence from other countries. A conceptual framework then draws two distinctions: one contrasting the relative impact of lifestyle habits with the use of health care on health outcomes; and the other seeking to quantify the importance of resources relative to behavioral factors in explaining differential outcomes. The literature to date has been more successful in documenting health inequalities than in explaining why these inequalities persist.