Following an introduction dealing with the developments which led to the discovery of the supplementary motor area (SMA), its anatomy and neuronal connections are described. Subsequently, a report concerning the pathology of the SMA (epilepsy, tumours, ablations, defects and possible neuroleptic effects) and related experimental findings is presented. This is followed by a description of the motor function, and especially the speech motor function of the SMA. The article concludes with an exposition of Sir John C. Eccles' interpretation of the SMA as providing the basis for a naive solution of the mind-body dualism.