Sleep-disordered breathing (mainly obstructive sleep apnea [OSA]) and COPD are among the most common pulmonary diseases, so a great number of patients have both disorders; this "overlap syndrome" causes more severe nocturnal hypoxemia than either disease alone. This common combination of OSA and COPD has important implications for diagnosis, treatment, and outcome. Specifically, patients with COPD and OSA have a substantially greater risk of morbidity and mortality, compared to those with either COPD or OSA alone. Only now are the interactions between these 2 systemic diseases being determined and appreciated. Many questions remain, however, with regard to disease definition, prognosis, and optimal treatment. Treatment currently consists of continuous positive airway pressure, and oxygen as needed. Noninvasive ventilation may be helpful in overlap syndrome patients, but this has not yet been well studied.