This study estimates the prevalence, assesses predictors and evaluates factors associated with concurrent and simultaneous use of drugs and alcohol in the United States population. Using data from the 2000 National Alcohol Survey (n=7612), respondents were asked if they used specific drugs in the last 12 months. Current drinkers who reported using each type of drug were asked if they used alcohol and the drug at the same time. Approximately 10% reported using marijuana in the last 12 months (concurrent use); 7% reported drinking alcohol and using marijuana at the same time (simultaneous use). Approximately 5% of current drinkers reported using drugs other than marijuana in the last 12 months; 1.7% reported drinking alcohol and using drugs other than marijuana at the same time. Being younger, having less than a high school education, not having a regular partner and having heavier drinking patterns were associated with using alcohol and marijuana simultaneously. Simultaneous use of marijuana and alcohol as well as other drugs and alcohol were significantly related to social consequences, alcohol dependence, and depression. These results mirror clinical populations in which increasingly younger clients report use of alcohol and drugs and need treatment for both.