Essentially all characters show some phenotypic variation, either between individuals within a population and/or between populations. Such variation is probably due to both genetic and environmental differences. Sorting out the nature of these genetic differences, the relative importance of genetic versus environmental factors, and how this phenotypic variation translates into evolutionary change is the domain of quantitative genetics. As such, this branch of genetics provides the theoretical foundations for plant and animal breeding and for much of human and evolutionary genetics. It is generally impossible to determine the genotypes at all relevant loci influencing a trait simply from an individual’s phenotype. Fortunately, the relative importance of genetic and environmental factors for a particular trait can be estimated from the phenotypic resemblance between relatives. While quantitative genetics has historically relied entirely on phenotypic information, more recently the tools of molecular biology are being applied in attempts to locate quantitative trait loci (QTLs), loci at which segregation contributes to the observed character variance. Identifying QTLs will augment, rather than supplant, phenotypicmeasures, and it is straightforward to incorporate such genotypic information into aquantitative genetic analysis.