There is little consensus about the meaning of the terms "health disparities," "health inequalities," or "health equity." The definitions can have important practical consequences, determining the measurements that are monitored by governments and international agencies and the activities that will be supported by resources earmarked to address health disparities/inequalities or health equity. This paper aims to clarify the concepts of health disparities/inequalities (used interchangeably here) and health equity, focusing on the implications of different definitions for measurement and hence for accountability. Health disparities/inequalities do not refer to all differences in health. A health disparity/inequality is a particular type of difference in health (or in the most important influences on health that could potentially be shaped by policies); it is a difference in which disadvantaged social groups-such as the poor, racial/ethnic minorities, women, or other groups who have persistently experienced social disadvantage or discrimination-systematically experience worse health or greater health risks than more advantaged social groups. ("Social advantage" refers to one's relative position in a social hierarchy determined by wealth, power, and/or prestige.) Health disparities/inequalities include differences between the most advantaged group in a given category-e.g., the wealthiest, the most powerful racial/ethnic group-and all others, not only between the best- and worst-off groups. Pursuing health equity means pursuing the elimination of such health disparities/inequalities.