Obstructive Sleep Apnea Syndrome (OSAS) is a sleep disorder characterised by repetitive episodes of upper airway obstruction (apnea) or reduced airflow (hypopnoea) despite persistent respiratory effort. Apnea is defined as the cessation of breathing for at least 10 seconds during sleep, while hypopnoea is defined as at least 30% reduction in airflow for 10 seconds associated with oxygen desaturation and sleep fragmentation. The presence in the general population is about 4%. The principal symptoms are: excessive daytime sleepiness (EDS), snoring, dry throat, morning headache, night sweats, gastro-esophageal reflux, and increased blood pressure. Long term complications can be: increased cardio-cerebrovascular risk and cognitive impairment such as deficiency in attention, vigilance, visual abilities, thought, speech, perception and short term memory. Continuous Positive Airway Pressure (CPAP) is currently the best non-invasive therapy for OSAS. CPAP guarantees the opening of upper airways using pulmonary reflexive mechanisms increasing lung volume during exhalation and resistance reduction, decreasing electromyografical muscular activity around airways. The causes of cognitive impairments and their possible reversibility after CPAP treatment have been analysed in numerous studies. The findings, albeit controversial, show that memory, attention and executive functions are the most compromised cognitive functions. The necessity of increasing the patient compliance with ventilotherapy is evident, in order to prevent cognitive deterioration and, when possible, rehabilitate the compromised functions, a difficult task for executive functions.