Obesity is a metabolic disorder affecting millions of people worldwide. The World Health Organization defines overweight as a BMI ≥ 25 kg/m2 and obesity as a BMI ≥ 30 kg/m2.1 Obesity is strongly associated with a variety of health problems, including respiratory impairment and sleep-disordered breathing. The term “sleep-disordered breathing” encompasses a spectrum of abnormalities. Obstructive sleep apnea (OSA) is characterized by repetitive episodes of apnea or hypopnea due to intermittent upper-airway obstruction. Obesity-hypoventilation syndrome (OHS), sometimes referred to as Pickwickian syndrome, is defined as chronic hypercapnia and hypoxia in an obese patient who lacks other known causes of chronic alveolar hypoventilation. OSA and OHS may exist simultaneously in individual patients, but OHS generally occurs in individuals with more extreme obesity. In this article, we review the physiologic characteristics of respiratory function in obesity, OSA, and OHS.