Chronic obstructive pulmonary disease (COPD) is a common chronic airway disease with progressive respiratory system and is characterized by obstruction of lower airways and decreased airflow due to an abnormal inflammatory response of the lungs exposing to noxious particles or gases. Obstructive sleep apnea (OSA) is a condition characterized by increased upper airway resistance associated with an intermittent decrease or absence of inspiratory air flow. COPD and OSA are two diseases that often coexist within an individual. This coexistence is known as overlap syndrome. Overlap patients might present both upper and lower airway obstruction and a reduction in respiratory drive, contributing to more nocturnal desaturation and hypercapnia, which potentially increases the risks of cardiovascular events, pulmonary hypertension and heart rhythm disturbances. In addition, COPD exacerbation rate might increase among overlap patients compared to COPD alone. The association of COPD and OSA will be reviewed in this talk.