Non-adherence to a drug therapy is often the reason for not achieving the therapeutic goals in patients. Measuring drug adherence is an important intervention to understand patients’ adherence patterns and behavior as well as to provide supportive measures to enhance the patient adherence to a prescribed drug therapy. A variety of different adherence measurement systems (AMS) exist that have been proven valid even though there is not one single AMS considered to be a gold standard. Some AMS allow through interactions with the physician or pharmacist and the patient to initiate corrective interventions, mainly in the form of alert and reminder systems. When applied to patients with several morbidities, co-morbidities and disabilities, appropriate AMS still remain a challenge.