Behavior and molecular genetics informs knowledge of the etiology, structure, and development of the Five Factor Model (FFM) of personality. Behavior genetics uses quantitative modeling to parse the relative influence of nature and nurture on phenotypes that vary within the population. Behavior genetics research on the FFM has demonstrated that each domain has a heritability (proportion of variation due to genetic influences) of 40–50%. Molecular genetic methods attempt to identify specific genetic mechanisms associated with personality variation. To date, findings from molecular genetics are tentative, with significant results failing to replicate and accounting for only a small percentage of the variance. However, newer techniques hold promise for finding the “missing heritability” of FFM and related personality domains. This chapter presents an overview of commonly used behavior and molecular genetic techniques, reviews the work that has been done on the FFM domains and facets, and offers a perspective for future directions.