Obesity is a national epidemic with rates that have dramatically increased over the past 2 decades. Currently, 34% of adults and 17% of children in the United States are obese. In addition to the well-established risks of heart disease, stroke, diabetes, and some cancers, obesity is driving a new epidemic in sleep-disordered breathing in the form of obesity hypoventilation syndrome (OHS). OHS is a syndrome characterized by a triad of obesity, daytime hypoventilation, and sleep-disordered breathing. It is becoming more readily recognized in the hospital and intensive care setting as patients present with acute on chronic hypercapnic respiratory failure. This review discusses the epidemiology, clinical presentation, pathophysiology, and treatment of chronic and acute or decompensated OHS.