Genetic influences on psychiatric disorders are well established. However, localization of the genes responsible for these effects has proved extremely difficult. One emerging strategy that may circumvent some of these difficulties is the use of quantitative risk factors, or endophenotypes, which are correlated with disease and may be closer to underlying genetic liability and to gene action than are diagnostic phenotypes. Genetic studies of quantitative endophenotypes require different sampling and analysis strategies than studies of disease state. The rationale for using quantitative risk factors as indicators of disease liability and the optimal study design for localizing genes influencing such risk factors are discussed.