This chapter summarizes current knowledge about changes in pulmonary physiology associated with obesity. It describes the pathophysiology of obstructive sleep apnea (OSA) and obesity hypoventilation syndrome (OHS). In obese individuals with OSA, increased soft tissue deposition in the pharyngeal region and tongue contributes to a decreased upper airway size. The polysomnography (PSG) remains the standard for diagnosis and assessment of the severity of OSA. Expert consensus concluded that the degree of peri-operative risk for patients with OSA depends on the severity of the OSA and the type of surgery. Obesity hypoventilation syndrome is a diagnosis of exclusion and requires the absence of other reasons for chronic hypoventilation such as chronic lung or neuromuscular disease. Additional treatment options for OHS similar to those for OSA are available including pharmacotherapy. The significant potential cardio-pulmonary co-morbidities present in OHS patients mandate a high index of suspicion to identify these patients preoperatively.