Successful pharmacotherapy requires patient compliance with the therapeutic regimen. The rate of non-compliance has been reported to be as low as 15% in some studies and the costs of non-compliance have been estimated to be between $396 and 792 million. Compliance, adherence, and persistence are defined. Subjective and objective methods used to measure compliance are reviewed. Subjective methods include patient self-reporting and physician estimates; these methods are generally inexpensive but have been shown to be unreliable. Objective methods include pill counting, electronic monitoring devices, and data reviews; these methods vary in cost and reliability.