This report presents descriptive information gathered from interviews with 945 youth aged 10-17 entering Texas Youth Commission (TYC) facilities in 1989 and focuses on both the criminal careers of the youth and the relationship between delinquency and substance use. Eighty-one percent of TYC youth reported illicit substance use with a median age of first use between the ages of twelve and thirteen. To analyze the delinquent careers of the youth, researchers interviewed them about their history of delinquent behavior. TYC youth were then classified into four groups based on their reported substance use habits over the previous year: (1) non-heavy substance users; (2) heavy users of alcohol only; (3) heavy users of illicit drugs only; and (4) heavy users of both alcohol and other drugs. The authors then drew comparisons between heavy users and non-heavy-users on such factors as early delinquency, criminal patterns, rates of illegal income, and gang participation. Youth were ranked by their pattern of crime and these crime patterns, or clusters, were associated with different kinds of substance use and sociodemographic characteristics. Finally, the relationship between crime and substance use was addressed with several hypotheses on how drugs and delinquency are related. (RJM)