Over the past two and a half decades, distinct approaches have been taken to defining and measuring health inequalities or disparities and health equity. Some efforts have focused on technical issues in measurement, often without addressing the implications for the concepts themselves and how that might influence action. Others have focused on the concepts, often without addressing the implications for measurement. This chapter contrasts approaches that have been proposed, examining their conceptual bases and implications for measurement and policy. It argues for an approach to defining health inequalities and health equity that centers on notions of justice and has its basis in ethical and human rights principles as well as empirical evidence. According to this approach, health inequality or disparity is used to refer to a subset of health differences that are closely linked with—but not necessarily proven caused by—social disadvantage. The term “inequity,” which means injustice, could also be used, but arguments are presented for using it somewhat more sparingly, for those inequalities or disparities in health or its determinants that we know are caused by social disadvantage.