Sleep and dreaming have been sources of mystery and fascination since biblical times. The function of sleep remains an enigma and only in recent years there has been research into various aspects of sleep and its related disorders. 'Obstructive Sleep apnea' (OSA) is a disorder characterized by abnormal breathing in sleep and sleep fragmentation. OSA is increasingly being recognized as important heath issue in the last two or three decades, it is characterized by repetitive, partial or a complete collapse of upper airway during sleep, causing impaired gaseous exchange and sleep disturbance.Aim of this book is to review the epidemiology, diagnosis/ evaluation, pathophysiology, etiology and risk factors, outcomes and various treatment modalities for OSA.