The concept of entropy, originally derived from thermodynamics, has been successfully applied to EEG analysis. Various entropy algorithms have been used in clinical studies, but until now a commercially available monitor exists only for spectral entropy. By calculating two distinct values for the EEG dominated part of the spectrum (state entropy, SE) and the total spectrum (response entropy, RE), the M-Entropy module claims to provide useful information regarding the cortical state of the patient as well as an indirect measure of adequacy of analgesia. Generally, entropy has been studied for quantification of anaesthetic drug effect for various GABA-ergic i.v. induction agents and volatile anaesthetics like propofol and sevoflurane and overall was found comparable to the current clinical gold standard bispectral index (BIS). Entropy guidance may not be used during ketamine or nitrous oxide administration, since there is no reliable correlation to the patient's state of consciousness. The usefulness of RE as a surrogate for increased EMG activity due to painful stimulation has not been proven so far.