An estimated 14 million Americans are afflicted with COPD and are at risk for significant abnormalities in gas exchange and ventilation that are exacerbated by sleep. In addition 10-15% of COPD patients concomitantly suffer from sleep apnea. The term "Overlap Syndrome" was originally coined by Flenley to describe the relationship between COPD and sleep apnea. Patients with overlap syndrome are characterized by having lower PaO2 during wakefulness, higher PaCO2, elevated pulmonary artery pressure and more significant episodes of nocturnal hypoxemia than sleep apnea patients without COPD. COPD and sleep apnea have long been individually recognized for having significantly detrimental affects on the respiratory physiology of patients. The mechanisms of both diseases compromise the gas exchange, oxygenation, and overall mortality and morbidity in the affected patients. While both of these diseases individually represent significant detriment to affected patients, the combination of these two diseases has been shown to have an even more profound affect on patients' oxygenation, gas exchange, and breathing patterns. As our understanding of the physiological processes of sleep develops, the relationship between obstructive sleep apnea and obstructive lung disease has become progressively more apparent. Identification and appropriate management of these patients is particularly important because the 5 year survival of patients with overlap syndrome is lower than that of patients with sleep apnea alone as shown in prospective trials.