Background: An achievement of modern anaesthesia is the ability to monitor its depth. Various methods have been described. More than 150 years ago, Snow and Guedel defined four stages of ether anaesthesia based on somatic muscle tone, respiratory parameters and ocular signs. Unfortunately, this method does not satisfy the objectives of modern anaesthesia with the presence of new anaesthetic drugs, complex and combined anaesthesia techniques, and increasingly older and sicker patients. During the last 15 years, more sophisticated depths of anaesthesia monitors have become available. These are based on electroencephalogram (EEG) or auditory-evoked potentials (AEPs). Both EEG and AEP transform biological signals that are related to changes in brain activity observed during anaesthesia. BIS and Spectral Entropy, both based on the EEG, were among the first commercialised depths of anaesthesia monitors. Sophisticated algorithms were used to express modifications of the EEG that are related to changes in brain status during anaesthesia. Some of these algorithms are accessible to a large public; some of them are kept secret. It was claimed that numerical scales recommended for adequate depth of anaesthesia were independent of anaesthetic drugs and patients' age. Knowing that primo: anaesthetic agents have different sites of action and diverse impacts on brain activity, and secondo: EEG patterns change with increasing age, it seems justified to investigate the influence of these factors on clinical utility of simplified EEG monitors. The aim of this work was to address these issues. Methods: Three studies were performed in which the influence of co-administration of opioids, chronic nicotine intake and increasing age on indices of BIS and Spectral Entropy were investigated. In these studies hypnotic component of anaesthesia was assured by strictly controlled administration of propofol. Sedation status, including loss of consciousness, was clinically evaluated using Observer's Assessment of Alertness/Sedation Scale. Indices of EEG monitors at loss of consciousness were recorded. Results: In the first study we observed that in the presence of an opioid loss of consciousness occurred in significantly higher BIS values and lower propofol concentration compared with placebo. In the second trial significantly higher values of BIS at baseline, at 0.7 g.ml-1 and 1.1 g.ml-1 propofol concentrations were found in smokers as compared with no-smokers, furthermore smokers lost consciousness at lower BIS values and higher propofol concentrations. In the third study loss of consciousness indices of BIS and Spectral Entropy were significantly higher in elderly ( 65yr) compared with young ( 40yr) patients. With all these monitors only a minority of elderly patients lost consciousness within recommended safety limits. Implications: All three investigated factors; opioid, chronic nicotine intake and increasing age influenced indices of simplified EEG monitors. Although clinically the hypnotic effect of propofol is enhanced by analgesic concentrations of -agonist opioids, the BIS does not show this increased hypnotic effect. Moreover the correlation between hypnotic effect of propofol and BIS is influenced by chronic nicotine intake. In adults undergoing propofol induction BIS, state entropy and response entropy at loss of consciousness are significantly affected by age. Further studies are needed to investigate the impact of these factors on EEG indices during general anaesthesia, but in our opinion they should be taken into account in the calculation of new software versions of these monitors.