Research on health differences by social class and economic status dates back to the mid-19th century. Early European publications examined differences in health by social class; then, during the next 100 years, the US produced the bulk of the evidence describing persistent and substantive differences in health across a broader range of social factors. During this descriptive period, scientists and clinicians came to realize that the differences noted in health care settings resulted from an array of structural, environmental, and social factors that spanned well beyond individual patients or the health care system. These broader factors are directly related to historical and current systemic inequities disadvantaging multiple population groups. The US is a fitting setting for this descriptive work, given its particularly stark social and economic inequities despite unparalleled national spending on research and health care. More recently, this research has evolved to explore underlyin...