Social class inequalities in health and mortality have become an increasingly prominent topic of study among sociologists, demographers, economists, and social epidemiologists. Considerable progress has been made in documenting such inequalities in a wide variety of settings using multiple measures of health and all-cause and cause-specific mortality. That social class inequalities are pervasive is now well established, but factors that underlie them are less clearly understood. This review discusses various measures used to define social class in studies of health inequalities. It then reviews the literature on patterns of these inequalities in developed countries as well as their potential explanations. Promising new research approaches include those that employ a life course perspective in the study of health inequalities and those that integrate multiple levels of analysis, including biological pathways that are likely to be involved in translating cumulative adversity to poor health.