Poor compliance with prescribed drug regimens is a widespread phenomenon which results in decreased efficacy and is often associated with increased medical expenditures due to treatment failure and/or adverse events. This review examines the issues relating to the measurement of compliance and the clinical and economic consequences of noncompliant drug taking behavior. The results of a literature review, identifying pharmacoeconomic evaluations which measured compliance, are presented. It concludes by highlighting the importance of accounting for poor compliance in pharmacoeconomic evaluations and suggests ways in which compliance may be incorporated in economic models.