In medicine, compliance (also adherence, capacitance) describes the degree to which a patient correctly follows medical advice. Most commonly, it refers to medication or drug compliance, but it can also apply to other situations such as medical device use, self care, self-directed exercises, or therapy sessions. Both the patient and the health-care provider affect compliance, and a positive physician-patient relationship is the most important factor in improving compliance, although the high cost of prescription medication also plays a major role. Compliance is commonly confused with concordance, which is the process by which a patient and clinician make decisions together about treatment. Worldwide, non-compliance is a major obstacle to the effective delivery of health care. Estimates from the World Health Organization (2003) indicate that only about 50% of patients with chronic diseases living in developed countries follow treatment recommendations. In particular, low rates of adherence to therapies for asthma, diabetes, and hypertension are thought to contribute substantially to the human and economic burden of those conditions. Compliance rates may be overestimated in the medical literature, as compliance is often high in the setting of a formal clinical trial but drops off in a “real-world” setting. Major barriers to compliance are thought to include the complexity of modern medication regimens, poor “health literacy” and lack of comprehension of treatment benefits, the occurrence of undiscussed side effects, the cost of prescription medicine, and poor communication or lack of trust between the patient and his or her health-care provider. Efforts to improve compliance have been aimed at simplifying medication packaging, providing effective medication reminders, improving patient education, and limiting the number of medications prescribed simultaneously.