The gap between best care and usual care is large for many important diseases. In particular, poor adherence remains a significant, inadequately addressed, cause of the care gap. About half of all patients with chronic diseases stop refilling prescriptions by one year. Several effective interventions are available and adaptations of clinical trials practices offer promise for further improvement. Poor adherence is a remedial problem in healthcare quality and its improvement and accountability offer shared opportunities for providers and patients. There is a large gap between best care, defined as the optimal use of proven efficacious therapies in whole populations at risk from any disease, and usual care, the actual level of efficacious care being provided (Montague et al. 1997). This gap in patient care has four main causes: diseases may not be diagnosed, efficacious therapies may not be prescribed, access to therapy may be restricted or patients may not adhere to prescriptions. Irrespective of causation, the ultimate result of care gaps is the same--less than optimal clinical outcomes and associated lost opportunities for improved quality of life and productivity. Systematic approaches to improving prescribing practices are increasing, and there is much debate around improving patients' access to care. Poor diagnosis is judged to be relatively uncommon, leaving decayed adherence as the major under-addressed cause of care gaps and a major opportunity for improvement. This paper reviews the scope and causation of sub-optimal adherence, evaluates improvement strategies and explores a best-practice benchmark.