It is well known that disease does not affect the population equally. Assessing variation in the rates of disease according to demographic factors such as sex and race or ethnicity is the basis of epidemiologic research and affects clinical and public health practice. The basis for such differences — in particular, among self-identified racial and ethnic groups — has recently been the focus of heightened discussion. The concept of race has perhaps triggered the greatest controversy, since it has been used historically to provide support for the mistreatment of one group by another. Also, the assignment of persons of mixed ancestry to one group (e.g., the “one-drop rule” for blacks) instead of recognizing their mixed ancestry undermines any simple formulation of racial categories. At the same time, numerous genetic studies of populations have identified genetic differences among people from different continents that often coincide with racial definitions, although within any population, there remains a great deal of genetic variation.