Research on the relationship between psychoactive substance dependence and personality disorder has been difficult to interpret. Among the reasons for this difficulty are (1) shifts in conceptualizations of these two classes of disorders, (2) markedly different findings related to age, sex, other demographic variables, and stage of illness, (3) overlapping of diagnostic criteria, and (4) instability of Axis II diagnoses over time in these populations. Personality disorder may be preexisting, but not necessarily predisposing. Nevertheless, Axis II diagnoses can be made in alcohol and other drug dependent patients, and are present in these populations at higher prevalence rates than in the general public. Although amelioration of personality disorder will not in itself necessarily correct substance dependence, Axis II diagnoses are of considerable importance in treatment planning and prognosis. Recent and future research on the genetic typology of alcoholism may shed further light on the complex relationship between genetics, physiology, personality, and addiction.