Epidemiological research provides information essential for defining the scope of the problem by identifying populations at risk. Epidemiological data on trends in illicit drug use and abuse over time help to measure the effectiveness of the national drug control program. Epidemiological research provides insights into the etiology of drug initiation and use (Chapter 5). Additionally, epidemiological research provides information on the nature and extent of the multiple consequences of drug abuse (Chapter 7). Data on drug availability and demographics allow prevention and treatment programs to target the needs of those populations identified as at risk for increased alcohol and illicit drug use.