Obesity hypoventilation syndrome (OHS) is defined by the triad of obesity (BMI ≥ 30 kg/m2), daytime hypoventilation (Paco2 ≥ 45 mm Hg), and sleep-disordered breathing in the absence of other causes of hypercapnia, such as severe obstructive lung disease, severe interstitial lung disease, neuromuscular disease, and chest wall deformities. Although approximately 90% of patients with OHS will have concomitant OSA, roughly 10% do not have sufficient hypopneas or apneas to meet criteria for OSA. These patients are instead found to have worsening of hypoventilation during sleep, particularly rapid eye movement sleep. Furthermore, concurrent OSA and hypercapnia are by no means pathognomonic for OHS. Indeed, OHS is a diagnosis of exclusion when the two are coexistent. A significant portion of patients with both OSA and hypercapnia may have concomitant COPD, a condition known as the overlap syndrome.