Since two decades, sleep breathing disorders are more wisely recognized by the Belgian medical community. Among these, the Obstructive Sleep Apnea Syndrome (OSA) is the best known but its frontiers with others syndromes such as the Upper Airway Resistance Syndrome (UARS), the Central Sleep Apnea Syndrome (CSAS) or the Overlap Syndrome are still matter of discussion. Its causes are plurifactorial, and many recent publications draw the attention to its long term effects in the cardiovascular and neuropsychiatric fields. This article summarizes the present definitions and features associated with OSA, from clinical and neurophysiological perspectives, and the different consequences to which untreated or underdiagnosed patients are exposed.