Lack of a brief empirically-verified screener for problematic and potentially problematic cannabis use has hampered implementation of a proactive opportunistic cannabis screening and early intervention (SEI) initiative in New Zealand. Addressing this instrumentation need was the primary aim of this thesis. This thesis also investigated the nature, prevalence, severity, and natural history of cannabis-related problems among a heterogeneous sample of 212 ‘at risk’ adolescent and adult users recruited from the community. In a 3-phase developmental design the CUPIT question candidates were first generated employing an Expert Panels methodology. The CUPIT structure, crosssectional, and longitudinal psychometric properties were then systematically tested, incorporating measures of cannabis-related pathology and DSM-IV/ICD-10 diagnoses of cannabis use disorders as criterion standard. High levels of cannabis consumption and related health and psychosocial problems reported portrayed a highly-disordered sample, most marked among adolescents. DSMIV/ICD-10 diagnoses were almost universal with no significant adolescent/adult differences in dependence symptoms count or severity. The two CUPIT subscales (Impaired Control, Problems) derived from principal components analyses exhibited good test-retest and internal consistency reliability and highly significant ability to discriminate diagnostic subgroups along the severity continuum (nonproblematic, risky, problematic use). At the 12-month follow-up, 194 adolescents and adults reported significantly increased cannabis consumption (adolescents), symptoms, and dependence severity. Baseline CUPIT subscale scores demonstrated highly significant longitudinal predictive utility for respondents’ diagnostic group membership, health and psychosocial problems, and significantly improved prediction of other measured outcomes in conjunction with age and gender. ROC analyses identified a CUPIT score of 12 to be the optimal cut-point for maximum sensitivity for both currently diagnosable cannabis use disorder and those ‘at risk’ in this sample. The empirical findings of this thesis research provide a compelling rationale for systematic implementation of opportunistic SEI among consumers of publicly-funded health and social services in New Zealand. Data confirmed that the vast majority of those needing help are unaware, or do not perceive, they need help. This thesis argues that, facilitated by the CUPIT, reliable proactive detection and appropriate intervention for early-stage cannabis use problems has potential for enormous cumulative impact on public health gains and the individual’s quality of life.