Adolescents were administered a modified version (version 3.0) of the World Health Organization Composite International Diagnostic Interview (CIDI), a fully structured interview administered by trained lay interviewers to generate DSM-IV diagnoses.28 The modifications to the CIDI were conducted following a standard 4-step process of instrument development proposed by experts in survey methods.24 First, diagnostic sections of the adult CIDI were reviewed to determine whether assessment for particular disorders should be removed based on low presumed prevalence in adolescents. Second, the language in the remaining CIDI sections was modified to enhance comprehension with adolescents using an iterative process. Third, CIDI modules were modified in content to make them more germane to the contexts and experiences of adolescents. The most common change of this type required altering references from adult contexts (eg, work life and parenting) to adolescent contexts (eg, school life and peer relationships). Fourth, the finalized revision of each diagnostic module was reviewed for meaning, logic, and comparability to the adult version. Each diagnostic section was then systematically piloted to test the flow and timing among adolescents, with subsequent modifications to reduce the length of the diagnostic sections. Lifetime substance use disorders assessed in the CIDI included alcohol and drug abuse, and individuals fulfilling abuse criteria were administered questions concerning dependence criteria. This report therefore presents both categories of abuse (with or without dependence). In light of differences in the pertinence of diagnostic categories for nicotine, trajectories for this substance will be addressed in a separate manuscript. In addition to DSM-IV diagnoses, all respondents were asked about the use of diverse substances and their age at which specific stages of use first occurred. For alcohol, the lifetime use question concerned age at which respondents first (if ever) had a drink with alcohol, specified as beer, wine, wine coolers, and hard liquor (eg, vodka, gin, whiskey, and mixed drinks). First regular use of alcohol was defined as the age at which the respondent first had at least 12 drinks within a single year. For illicit drugs, participants were asked their age at which they had first (if ever) consumed a range of specific substances, including marijuana or hashish; cocaine in any form (eg, powder, crack, freebase, coca leaves, or paste); tranquilizers, stimulants, pain killers, or other prescription drugs either without the recommendation of a health professional or for any reason other than a health professional said they should be used; and heroin, opium, glue, lysergic acid diethylamide, peyote, or any other drug. Participants were also asked about the first time they had an opportunity to drink alcohol or use drugs, regardless of whether they used them. Opportunity to use was defined as when someone either offered them alcohol or drugs or when the individual was present when others were using and could have used if he or she wanted to. The age at first opportunity to use substances was recorded separately for alcohol and drugs. In light of the high availability of legal substances, such as alcohol, only opportunity to use illicit drugs is examined in this investigation.