This chapter provides a brief overview of quantitative-genetic theory. Quantitative-genetics provides important tools to help elucidate the genetic underpinnings of behavioral and neural phenotypes. This information can then provide substantial insights into the previous evolutionary history of a phenotype, as well as into brain-behavior relationships. The most often employed crossbreeding designs are the classical Mendelian cross and the diallel cross. The information rendered by the former is limited to the two parental strains used and cannot be broadly generalized. The principal usefulness of this design is for testing whether a given phenotype is influenced by either one gene or by more genes. The diallel cross renders more generalizable information, the more so if many different strains are used, such as estimates of genetic correlations. To estimate the latter, correlations between inbred strain means may provide a helpful shortcut. Some commonly encountered mistakes in the interpretation of the results of quantitative-genetic studies are presented and explained.