The development of social epidemiology and medical sociology over the last half of the 20th century, in which Leo Reeder played a central role, transformed scientific and popular understanding of the nature and causes of physical health and illness. Viewed in the early 1950s as shaped almost entirely by biological processes and medical care, physical health and illness are now understood to be as much or more a function of social, psychological, and behavioral factors. Utilizing a stress and adaptation conceptual framework, social epidemiology has identified a broad range of psychosocial risk factors for health, most notably: (1) social relationships and support; (2) acute or event-based stress; (3) chronic stress in work and life; and (4) psychological dispositions such as anger/hostility, lack of self-efficacy/control, and negative affect/hopelessness/pessimism, with new risk factors continuing to be identified. However, proliferation of risk factors must be balanced by conceptual integration and causal understanding of the relationships among them, their causes, and consequences. One source of such integration and understanding has been the rediscovery of large and persistent socioeconomic and racial-ethnic disparities in health. Socioeconomic position and race/ethnicity shape individuals' exposure to and experience of virtually all known psychosocial, and well as many environmental and biomedical, risk factors, and these risk factors help to explain the size and persistence of social disparities in health. Improving the socioeconomic position of a broad range of disadvantaged socioeconomic and racial-ethnic strata constitutes a major avenue for reducing exposure to and experience of deleterious risk factors for health, and hence for improving the health of these groups and the overall population. This in turn requires better understanding of the macrosocial forces that influence the socioeconomic position of individuals.