While much is known about criminal behavior patterns of general population and delinquent youth, little is known about the pretreatment criminality of adolescent substance abuse treatment clients. This information is important because of substantial increases in adolescent substance abuse treatment admissions and because adolescents who use substances regularly are more likely than nonusers to be criminally involved. Descriptive information is presented on the lifetime patterns of substance abuse, criminal activity, and justice system involvement of 1,122 youth admitted to nine adolescent treatment programs located throughout the United States, including short- and long-term residential and outpatient programs. Study findings indicate that involvement in illegal activities, other than use and possession of substances, is common among adolescents entering treatment. Most clients in each modality were involved with the justice system prior to treatment, and pressure from the justice system to enter treatment was widely reported. While the severity and frequency of offenses committed by short- and long-term residential clients were considerably greater than outpatient clients, a sizable minority of outpatient clients had also committed serious offenses. In addition to differences among modalities, considerable variation in criminal involvement was found among programs within the same modality. Implications for treatment and research are discussed.