Using the National Survey of Homeless Assistance Providers and Clients (NSHAPC), this study examined predictors of alcohol and drug problems among a national probability sample of homeless and previously homeless adults in the United States. Consistent with research, about one third and one quarter of the respondents reported a current alcohol and drug problem, respectively. A larger proportion reported lifetime substance abuse problems. As hypothesized, males, younger respondents, veterans, and those with mental and physical health problems were at greater risk. Results reveal that substance abuse treatment and prevention programs for the homeless should take into consideration these characteristics to maximize effectiveness.