One of the most confusing things associated with the topic of depth of anesthesia monitoring is probably the numerous and mostly confusing “terminologies” that are being used. On many occasions, one would be using a certain terminology meaning something to him/her but could be perceived by the other person to mean something completely different. Chapter two will clarify all the three big terms being commonly used. Brain function monitors (BFM), Depth of Anesthesia (DOA) monitors, and processed Electroencephalography (pEEG) monitors are three terminologies often used interchangeably and frequently treated as synonyms when in fact they are not. A major problem facing all anesthesiologist is the fact that there is no recognized “comparator standard parameter” recommended by authorized bodies such as the National Institute for Health and Care Excellence (NICE) against which we can precisely evaluate the depth of anesthesia monitors performance.