Obstructive sleep apnea syndrome is widely prevalent problem with significant medical, psychological and social consequences that are deleterious to the quality of life and potentially lethal. It is only in the last two decades that this disorder has received the attention of clinical specialists, with the orthodontist playing a significant role in the management of the disorder. The Greek word "apnea" literally means without breath. Cessation of breathing for ten seconds or longer is termed as apnea. When thirty or more apneic episodes occur in the course of seven hours of sleep, resulting in excessive sleepiness during waking hours, a person is described as having sleep apnea syndrome. It is characterized by cessation of airflow created from breathing through upper airway while diaphragm movement continues.This book will deal with the etiology, pathophysiology, signs and symptoms, diagnosis and treatment of OSA syndrome.