Obesity Hypoventilation syndrome (OHS) is defined as the triad of obesity, daytime hypoventilation, and sleep disordered breathing in the absence of an alternative explanation for hypoventilation. Among patients with obstructive sleep apnea (OSA), those who are more obese and have more severe OSA are more likely to have the syndrome. It is unclear why some patients with OSA develop OHS and others do not. It is important to identify the syndrome early, as these patients may have a significantly increased risk of mortality if left untreated. The authors describe current understanding of the pathogenesis of the disorder and optimal treatment modalities.