The present volume is an attempt to synthesize, present, and argue for what has been learned and remains to be learned about the biological differences within and among human groups. Marks, a biologist as well as an anthropologist, avails himself of the data generated by molecular genetics about the hereditary composition of the human species. As it happens, genetics has undermined the fundamental assumptions of racial taxonomy, for genetic variation has turned out to be, to a large extent, polymorphism (variation within groups) rather than polytypy (variation among groups). Though populations at geographical extremes can be contrasted, the fundamental units of the human species are populations rather than races. Further, genetics provides little in the way of reliable biological history of our species, because human populations are culturally-defined, as well as biological, entities. And genetics has often been used as a scientific validation for cultural values - from the idea that there is indeed a small number of genetically distinct kinds of people ("races") to be identified to more pervasive suggestions about the relationship of genetics to behavior. In its presentation of the bio-cultural nature of human diversity as well as in its presentation of the history of the problem and the illusions embedded in that history, this will be a widely used textbook that fills a void in the literature of biology and of physical anthropology.