This chapter outlines the basic observations that must be faced in order to understand the genetics of personality disorders. It suggests a general functional approach based on the evolution of human brain functions that may be of general utility. The specification of personality disorders is better explained by comprehensive multidimensional personality inventories that measure normal and/or abnormal personality traits. Multidimensional inventories for assessing components of personality disorders have been developed to measure both abnormal traits and also traits that characterize the full range of variability in the general population. Twin studies of personality dimensions in the general population also demonstrate moderate heritability of normal personality traits. The estimates of heritability in twin studies are inflated by contributions from gene-gene and gene-environment interactions. A model of the evolution of brain functions may be the only way to make sense of the psychobiology of personality and psychopathology.