Nonadherence to treatment regimen is a prevalent problem of patients with chronic disorders. Approximately half of the patients with a chronic disease have problems following their prescribed regimen to the extent that they are unable to obtain optimum clinical benefit. This chapter reviews the state of knowledge regarding adherence to chronic disease regimens across the life span and demonstrates that the extent and nature of the adherence problems are similar across diseases, across regimens, and across age groups. Adherence to the commonly prescribed regimens is addressed, including pharmacological therapies, therapeutic diets, and therapeutic exercise. Randomized, controlled studies focusing on various educational, behavioral, cognitive, and affective interventions to improve adherence are included. Based on this review, further work is needed to better understand and improve adherence. New strategies for analysis and measurement will support these needed advances in the field of adherence.