Most common diseases and most quantitative traits that can be measured in human populations are complex genetic traits. That is, many genetic and nongenetic factors interact to determine the final phenotype, whether that phenotype is susceptibility to disease, or a quantifiable trait such as height, weight, serum cholesterol, or blood pressure. Identifying the genes that underlie the population variation in these phenotypes has been challenging. Recently, databases of common genetic variants, recognition of the patterns of genetic variation, and rapid genotyping methodologies have emerged, and the combination of these tools and resources will greatly facilitate genetic association studies, a potentially powerful method to map the genes for complex traits. However, care will be required in performing and interpreting these association studies. Until genome-wide studies are feasible, choosing candidate genes will be necessary. In addition, the choice of phenotype will likely influence the success of these gene mapping efforts. Finally, population genetic methods, including searching for genes under selection, may provide clues to the location of the genes for common disease and complex traits.