Anesthesia awareness, or "unintended intra-operative awareness" occurs during general anesthesia, on the operating table, when a patient has not had enough general anesthetic or analgesic to prevent consciousness or waking up during surgery According to international studies its global incidence ranges from 0 1 to 0 2% Its impact on people can be as severe as other traumatic experiences such as natural disasters, violent fights or sexual abuse. The prevalence of symptoms compatible with post traumatic stress disorder can be as high as 50%, after experiencing the awareness phenomenon. This paper reviews the main issues of the awareness phenomenon. (Rev Med Chile 2010; 138: 352-357).