Electrical stimulation of the human brain is one of the most exciting chapters in the story of experimental neurobiology. Since the first reported case of brain stimulation in a patient, by Bartholow in 1874, much progress has been made in this field. Today's methods of stimulation are much more subtle, and sophisticated methods for the assessment of the patient's experiences are used, as is shown by the following papers. The object of this issue is to provide an overview of the recent work in this field and to outline the influences it has, or should have, on our functional concepts of the human brain.