Awareness remains a serious complication of general anaesthesia with potential adverse psychological sequelae. Even during seemingly adequate general anaesthesia, implicit memory may be retained along with the ability to subconsciously process auditory stimuli. As a result behaviour may be modified and postoperative progress influenced. We shall discuss the structure of memory and the effects of increasing doses of general anaesthesia on cognitive processes. In addition methods of assessing the depth of anaesthesia will be reviewed.