Reviews longitudinal studies that have demonstrated that personality traits are stable in adulthood. Findings suggest that there are no age-related shifts in mean levels and that individuals maintain very similar rank ordering on traits after intervals of up to 30 yrs. It is concluded that these findings should be of interest to clinicians because they point to important similarities between normal personality and personality disorders, facilitate research on the psychological processes that maintain both adaptive and maladaptive traits, serve as a reminder that current problems in functioning may be the expression of enduring personality patterns, and foster more realistic expectations about how much therapeutic change is possible.