Non-adherence to medicines is common, with convincing evidence for adverse effects on patient health and costs to health systems. At least half of reported non-adherence is intentional. An extensive body of research suggests that, while patient characteristics may contribute to this behaviour, key influences are linked to beliefs and experiences of an illness and its medicines. Characteristics of the health system such as patient-practitioner relationships and access are also significant drivers. Inadvertent effects of some policies, such as co-payments, reduce adherence. Interventions to improve adherence have not utilized available research evidence fully and are not integrated into service delivery, so have been disappointing in producing sustained behaviour change. Policies relying on patients' adherence to medicines will not be as effective as hoped if adherence is assumed rather than supported. Substantial gains could be made by patients and health systems if patients, practitioners, researchers and policy-makers worked together to improve this crucial area of health behaviour.