Socioeconomic health inequality is substantial, ubiquitous, and persistent. This chapter reviews what is known about its causes in high-income countries and considers what can be known and needs to be known. Causal analyses have not yet delivered strong, consistent evidence that education, income, and wealth impact health in adulthood, but there is evidence that cash benefits paid to low-income households often improve infant and child health outcomes. Changes in adult health have large effects on income and wealth, and childhood ill-health both persists into adulthood and constrains economic outcomes in that phase of life. What can be known about the causes of health inequality is constrained by the limited scope for causal analysis to identify effects of socioeconomic exposures (including the relative concept of status) that potentially take their toll on health over the life course, cumulatively and multiplicatively. To reduce health inequality, its causes need not necessarily be known, provided health policies that improve the health of the socioeconomically disadvantaged can be identified and implemented. Political support for such policies may, however, depend on knowledge (or beliefs) about the causes of health inequality.