Obesity affects pulmonary function profoundly. Lung pathology is rarely found in obese people. Thus the functional abnormalities often observed in obese people (e.g., reduced lung volumes, increased airway resistance, hypoxemia and hypercapnia) are a consequence of the added load presented by chest wall adiposity to the respiratory system. This paper describes the mechanisms whereby obesity brings about these functional abnormalities. It then describes the two major syndromes associated with obesity, one, the obesity - hypoventilation syndrome and two, the obstructive sleep apnea syndrome, and links these syndromes to the chest wall pathology and physiology which are their cause.