This text examines the basic constructs of gender, sex, ethnicity, and race. These constructs are routinely used in the analysis of data in a number of fields, including epidemiology, health services research, sociology, and medical anthropology. Despite the widespread use of these terms, little thought has been given to what they really mean. What does it mean, for instance, to say that African Americans are at a higher risk of AIDS than other groups? Is this really a surrogate for sexual behaviour? For access to medical care? For an unknown genetic difference in the immune system? Since ethnicity/race is not a mutable characteristic, are we accomplishing anything by focusing on it as a risk factor or risk marker, or should we be examining its underlying meaning? Should we continue to utilize these constructs and, if so, how? What are the implications for intervention programs and intervention research? This text addresses these constructs of gender and ethnicity in a manner that challenges the conventional wisdom within the health sciences.