Daytime hypercapnia that develops in morbidly obese individuals in the absence of concurrent lung or neuromuscular disease is referred to as the obesity hypoventilation syndrome (OHS). The characteristic polysomnographic (PSG) abnormality is marked sleep hypoxemia. Although the likelihood of hypoventilation increases with increasing body mass index (BMI), it is too simplistic to think of this disorder arising merely from chest wall restriction due to excess weight. Rather, this is a disorder which emerges when the compensatory mechanisms that normally operate to maintain ventilation appropriate for the level of obesity are impaired. OHS develops from a complex interaction between abnormal respiratory function, sleep disordered breathing and diminished respiratory drive. Irrespective of the mechanisms underlying the development of this disorder, early recognition of the problem and institution of effective therapy is important to reduce the significant clinical and societal repercussions of OHS. While therapy directed at improving sleep disordered breathing is effective in reversing daytime respiratory failure, it is not universally successful and information regarding longer term clinical outcomes is limited. Attention to weight reduction strategies are also necessary to reduce comorbid conditions and improve quality of life, but data regarding how successful and sustained this is in obesity hypoventilation are sparse.