The P300 component studies, have been generated discussion about his source, cognitive meaning and clinical applications. Since the first point of view, different structures have been proposed as a probable source of P3a or P3b in oddball and memory task and in scalp and in a intracraneal recordings. There are many papers that lucubrate about cognitive meaning of P300 and the applications of this component in the diagnostic process of psychiatric pathology. We make critic analysis and different points of view about this component.