The National Household Survey on Drug Abuse (NHSDA) is the primary source of statistical information on the use of illegal drugs by the U.S. population. This reports presents information from the 2000 NHSDA on national estimates of the extent of substance dependence, abuse, and treatment and provides national estimates of what has been called the treatment gap, those individuals needing treatment and not receiving treatment, and the need for and receipt of treatment for an illicit drug problem. After an introduction, the report offers estimates of the prevalence and patterns of substance dependence and abuse in the Nation. Then, it provides estimates of the prevalence and patterns of the receipt of treatment for problems related to substance use. Finally, the report discusses the need for and receipt of treatment specifically for problems associated with illicit drug use. When reviewing the extent of substance dependence, abuse, and treatment the characteristics of the estimates include: age, gender, race/ethnicity, education, employment, and geographic area. Selected findings include: (1) in 2000, an estimated 14.5 million Americans aged 12 or older were classified with dependence on or abuse of either alcohol or illicit drugs; (2) among past year users of heroin in 2000, 60.1 percent were classified with dependence on or abuse of heroin; (3) adults in 2000 who first used drugs or alcohol at a younger age were more likely to be classified with dependence on or abuse of substances than adults who initiated at a later age; (4) an estimated 2.8 million people aged 12 or older received some kind of treatment for a problem related to the use of alcohol or illicit drugs in the 12 months prior to being interviewed in 2000; (5) in 2000, males were more likely to receive treatment compared with females among those aged 12 or older and those aged 12 to 17; (6) in 2000, the pattern in rates of treatment at specific locations for a problem with illicit drugs was similar to the pattern in rates of treatment at specific locations for a problem with alcohol; (7) in 2000, an estimated 4.7 million people aged 12 or older needed treatment for illicit drug abuse problems; and (8) among the 2.1 million individuals aged 12 or older who received alcohol treatment in the past year, a greater number received treatment at a self-help group than any other location. Figures, tables, references, and appendices A-F