Diagnostic classification systems have developed to the point at which the DSM-IV and ICD-10 are nearly identical, so that researchers and clinicians in different parts of the world have a common language for substance-dependence diagnoses. Despite the differences in nosology, the demographic correlates of alcohol and drug dependence are strikingly similar. Lifetime and 12-month prevalences are generally higher in men than in women, whites compared with nonwhites, younger compared with older cohorts, those with lower income levels and lower educational attainment, and those who have not been stably married (including those who have cohabited). NCS data indicated differences in risk factors for stages of drug use, arguing for separation of these in future analyses. Alcoholics are more likely to have another psychiatric disorder compared with their nonalcoholic counterparts, and ASPD, mania, and other drug dependence rank among those disorders most strongly associated with alcohol and drug dependence. Analyses from the NCS examining temporal ordering of diagnoses have focused attention on anxiety disorders in the cause of alcohol dependence. Depression, although not so strongly associated with substance dependence as clinical studies had led researchers to believe, nonetheless seems to be of etiologic interest as well, according to the NCS analyses. The cross-sectional results of the NLAES data on comorbidity between depression and alcohol and drug dependence have uncovered important new associations between gender, age, and depression and may yield further etiologic insights when age-of-onset data are taken into account.