Obesity is becoming a major medical concern in several parts of the world, with huge economic impacts on health- care systems, resulting mainly from increased cardiovascular risks. At the same time, obesity leads to a number of sleep-disordered breathing patterns like obstructive sleep apnea and obesity hypoventilation syndrome (OHS), leading to increased morbidity and mortality with reduced quality of life. OHS is distinct from other sleep- related breathing disorders although overlap may exist. OHS patients may have obstructive sleep apnea/hypopnea with hypercapnia and sleep hypoventilation, or an isolated sleep hypoventilation. Despite its major impact on health, this disorder is under-recognized and under-diagnosed. Available management options include aggressive weight reduction, oxygen therapy and using positive airway pressure techniques. In this review, we will go over the epidemiology, pathophysiology, presentation and diagnosis and management of OHS.