Despite the wide application of adherence as a concept, the definition, evaluation and improvement of the adherence to treatment by patients with chronic obstructive pulmonary disease (COPD) still present some challenges. First, it is necessary to clearly define the concepts of treatment adherence, compliance and persistence. Second, it is critical to consider the various methods of evaluating and quantifying adherence when interpreting adherence studies. In addition, the advantages and disadvantages of the different ways of measuring treatment adherence should be taken into account. Another subject of some debate is the number of variables associated with COPD treatment adherence. Adherence is a complex concept that goes beyond the dosage or the use of inhalation devices, and a number of variables are involved in determining adherence, from the clinical aspects of the disease to the patient’s confidence in the doctor’s expertise and the level of social support experienced by the patient. Notably, despite these challenges, the importance of adherence has been well established by clinical trials and routine clinical practice. The available evidence consistently shows the substantial impact that a lack of adherence has on the control of the disease and its long-term prognosis. For these reasons, the correct evaluation of therapeutic adherence should be a key objective in clinical interviews of patients. In recent years, various initiatives for improving adherence have been explored. All these initiatives have been based on patient education. Therefore, health care professionals should be aware of the issues pertaining to adherence and take the opportunity to educate patients each time they contact the health care system.