Increased attention has been focused on the use of racial and ethnic categories in public health research and surveillance in recent years. This attention, however, has done more to increase the collective awareness of the difficulties inherent in these classification schemes than to resolve either the theoretical or practical problems. It has been recognized for many years that health outcomes for a particular ethnic population must be seen as an interaction between genetically determined factors and socially mediated exposures. The operational meaning of these concepts remains poorly defined, however. Although the general biologic construct of race and ethnicity appears straightforward, appearance is deceiving and the technical requirements that allow formulation of a rigorous and objective working definition have never been fully developed. The social factors that influence ethnic health differentials are perhaps better understood in practical terms; measurement of many of the relevant variables remains difficult, however. In this article an attempt is made to outline the implications of the new developments in molecular biology for the concept of race, and to provide an illustrative example of the continued evolution of the social determinants of ethnicity.