Non-adherence to medicine is common in patients with chronic diseases, contributing to significant worsening of disease, increased mortality and health expenditure. Methods of measuring adherence include self report, prescription refill rates, biomarkers, electronic monitoring and therapeutic outcomes. Yet, no "gold standard" for assessing adherence and no consensus on what is an acceptable level exist. Physicians should be aware of non-adherence and, although it may not always be identical with the evidence-based regimen, they can facilitate good adherence by simplifying regimens and adapting treatments to the patient's lifestyle and preferences.