Autonomic nervous system (ANS) has been extensively explored in obstructive sleep apnea (OSA). Autonomic alterations in these patients have been described by means of several methods, evaluating ANS function both directly with microneurography and indirectly through baroreflex sensitivity (BRS, by the sequence method or the cross-spectral approach), heart rate variability analysis (HRV, both in the time and frequency domain) during sleep and wake, or conventional laboratory tests, including cold pressor test, hand grip test or measurement of urinary cathecolamine excretion. Several studies in OSA patients have shown ANS alterations, in particular sympathetic overactivity, both acutely during apnea events and chronically during the daytime, being both also involved in cardiovascular consequences of sleep disordered breathing. The association between OSA and sympathetic dysregulation suggests a dose response relationship between OSA severity and the degree of sympathetic overactivity and this association seems to be reversible as the treatment of OSA is implemented. Additionally ANS is involved in regulating visceral and humoral functions to maintain the body homeostasis and in reaction and adaptation to external and internal stressor stimuli. However, the vast majority of studies have focussed on cardiovascular alterations, which are easier to measure, somewhat neglecting the other functions regulated by ANS. More evidence is therefore needed to better characterize the impact that sleep disorder breathing may have on ANS both in the short and long term.