Molecular and genetic approaches now provide powerful tools for investigating the origin of human populations and the evolution of genes affecting both complex and monogenic traits. This allows a biologically appropriate classification of individuals and population groups based on genotypes and gene frequencies rather than appearance; this information will facilitate disease risk assessment. Many of the observed differences in gene frequencies between human populations may be accounted for by population movements, and it is difficult to assess how large a role natural selection has played in population differentiation. For several disease genes there is evidence that environmental agents, such as infectious pathogens, dietary factors, and environmental toxins, have been responsible for an increased frequency in some populations. Further examples of such selection may emerge from current efforts to define the genetic basis of many polygenic common diseases. We need more information on molecular genetic differences between human populations; we could obtain that information from the successful completion of the Human Genome Project and the proposed Human Genome Diversity Project.