As many as half of all patients do not adhere faithfully to their prescription-medication requirements, and the result is more than $100 billion spent each year on avoidable hospitalizations. David Cutler and Wendy Everett discuss how to improve medication adherence. Poor adherence to treatment regimens has long been recognized as a substantial roadblock to achieving better outcomes for patients. Data show that as many as half of all patients do not adhere faithfully to their prescription-medication regimens — and the result is more than $100 billion spent each year on avoidable hospitalizations. 1 Nonadherence to medication regimens also affects the quality and length of life; for example, it has been estimated that better adherence to antihypertensive treatment alone could prevent 89,000 premature deaths in the United States annually. 2 What is less clear is why adherence to the 3.8 billion prescriptions written . . .