Alcohol and tobacco consumption are correlated-smokers consume more alcohol than do nonsmokers and alcohol consumers smoke more than do teetotalers. In addition, heavy drinking tends to be associated with heavy smoking. A large majority of alcoholics, who by definition are heavy drinkers, smoke. A number of studies examining the effects of ethanol pretreatment or availability on tobacco consumption have demonstrated that ethanol potentiates tobacco consumption. Whether smoking potentiates alcohol consumption is not known. Possible mechanisms underlying the alcohol/tobacco association, including cross-tolerance between the two drugs, are discussed. Tobacco appears to counteract the deleterious effects of alcohol on some measures of performance. The association between alcohol and tobacco consumption may have some relevance regarding the issue of relapse to either one of the drugs. Finally, combined use of alcohol and tobacco presents greater risk of certain diseases than the sum of the excess risks of alcohol and tobacco considered individually.