In 1984 Putnam reviewed the literature on the psychophysiological investigation of multiple personality disorder (MPD). Since his review, a large number of studies have been conducted and reported in the literature and at professional conferences. Currently, psychophysiologic differences reported in the literature include changes in cerebral electrical activity, cerebral blood flow, galvanic skin response, skin temperature, event-related potentials, neuroendocrine profiles, thyroid function, response to medication, perception, visual functioning, visual evoked potentials, and in voice, posture, and motor behavior. We review the new research on the psychophysiological investigation of MPD from published, unpublished, and ongoing studies, and we attempt to place current findings into a conceptual framework. We have noted findings from unpublished and ongoing studies, and, perhaps unfortunately, they represent a large amount of the data presently available. We conclude with a critical analysis of current research methodology and suggestions for future research.