The development and psychometric evaluation of the Canadian Adolescent Gambling Inventory (CAGI) was undertaken in two phases. Phase I consisted of: (a) an examination of how problem gambling is conceptualized, defined and measured in the literature; and (b) the development of a new conceptual framework, definition and means of measurement. This phase of the research involved an extensive review of the literature, consultation with a panel of experts in the field and focus groups with adolescents. The result was the development of a new conceptual framework and operational definition and the development of a draft instrument for measuring problem gambling. Phase II of the project involved the fine-tuning and testing of the validity and reliability of the instrument developed in Phase I. This was accomplished by testing both an English and French version on a sample of adolescents drawn from school populations in Manitoba and Québec. Data collection included a pilot test with 195 students from Manitoba and 277 students from Québec. This was followed by a general school survey with 2,394 students, a retest of 343 students from the general school survey, and clinical validation interviews with 109 students who initially participated in the general school survey. The original Phase II research design proposed utilizing two external sources of data to interpret scale scores and establish cutscores for levels of risky gambling behaviour; namely youth in treatment for gambling problems and clinician's assessments. It is important to assess the classification accuracy of the instrument (i.e., sensitivity, specificity, positive and negative predictive values) for detecting 'problem gambling cases' against a reference standard such as a case assessed by an expert interviewer. During Phase II, we were unable to locate any 12–17 year olds in treatment for a gambling problem. As well, the clinical interviews with school students resulted in very few students being classified as problematic gamblers. Therefore, in the absence of external validation criteria and expert consensus, frequency distributions and measures of central tendency were used to determine 'abnormal' gambling behaviour for a school sample of gamblers. As such, cutscores and score interpretations provided by Phase II work were temporary. The results needed to be cross?validated with other relevant samples; particularly, samples that include youth with gambling problems. Phase III addressed the limitation of Phase II by reaching a new sample of youth who were at greater risk of having problems with gambling (e.g., adolescents who were receiving treatment for substance abuse or were receiving services from youth centres) or who were experiencing problems with gambling.