Life expectancy has always differed according to status in society, with a higher mortality among those of lower social status. Although cancer and cardiovascular diseases are more common as causes of death in rich than in poor societies, in industrialized countries the major causes of death are more common in those of lower social status. In this chapter, the magnitude of socioeconomic differences in health is examined using different measures of socioeconomic status, and methodological issues relating to these measures are discussed. Much of the discussion about social inequalities in health has been focused on the health disadvantage of those of lowest socioeconomic status. However, data from the Whitehall studies show that the social gradient in morbidity and mortality exists across employment grades in British civil servants, none of whom is poor by comparison with people in developing countries, suggesting that there are factors that operate across the whole of society. A number of potential explanations are considered here. The magnitude of socioeconomic differences in health varies between societies, and over time within societies. This suggests that identification of factors that influence socioeconomic status and health, and the pathways by which they operate, is an important public health task that could lay the basis for a reduction in inequalities in health.