Although the terms "health equity" and "health disparities" have become increasingly familiar to health professionals in the United States over the past two decades, they are rarely defined. Federal agencies have often defined "health disparities" in ways that encompass all health differences between any groups. Lack of clarity about the concepts of health disparities and health equity can have serious consequences for how resources are allocated, by removing social justice as an explicit consideration from policy agendas. This paper aims to make explicit what these concepts mean and to discuss what a life-course perspective can contribute to efforts to achieve health equity and eliminate health disparities. Equity means justice. Health equity is the principle or goal that motivates efforts to eliminate disparities in health between groups of people who are economically or socially worse-off and their better-off counterparts-such as different racial/ethnic or socioeconomic groups or groups defined by disability status, sexual orientation, or gender identity-by making special efforts to improve the health of those who are economically or socially disadvantaged. Health disparities are the metric by which we measure progress toward health equity. The basis for these definitions in ethical and human rights principles is discussed, along with the relevance of a life-course perspective for moving toward greater health equity.