Factors which can lead to awareness during surgery are considered. They are divided into those affecting induction and those affecting maintenance. Induction: The risk of a patient recovering consciousness while still paralyzed is greater if an ultrashort-acting intravenous anaesthetic is used. The time interval necessary before surgery may begin is governed by the need to build up a sufficient concentration of inhalation agent to take over from the waning effect of the intravenous agent. Maintenance: Three sources of trouble are distinguished, each leading to the patient receiving an anaesthetic mixture less potent than intended. (1) Ventilators may mix air or oxygen with the anaesthetic gas if they are not adjusted properly or if there are leaks. Ventilators are classified according to their propensities for doing this. (2) Some anaesthetic machines have pitfalls. (3) A circle system is shown to need time to acquire an adequate concentration of anaesthetic. The importance of high flowrates is stressed both at the start of an anaesthetic and also if the circuit should happen to be broken later on.