The authors describe the changes commonly occurring in the electroencephalogram during anaesthesia, and discuss their interpretation and significance. In particular the changes associated with hypoxia, hypotension, over-ventilation and interference with the cerebral blood supply are dealt with. The usefulness of the electroencephalogram in different circumstances is assessed including experimental work, and recent reports on recording evoked cortical potentials, and on recording during stereotactic surgery are mentioned. The technical problems of recording and instrumentation are outlined and the most useful systems indicated.