Modern anaesthesia has developed over 150 years. In spite of this, we still do not know how to explain the state of anaesthesia exactly, how to measure anaesthesia nor what the basic mechanisms are. In this paper, a model for the state of anaesthesia, is presented, and some aspects of basic mechanisms and monitoring are discussed. Anaesthesia modulates or blocks the somatic and autonomic responses to injury, mainly by postsynaptic inhibition of impulses. Empirical dosing and clinical monitoring are still the rule, computerized and electronic systems need to be further refined.