Those interested in the relationships between psychological and physiological functions will again and again be impressed by the fact that great individual differences and large situational variability are manifested in psychophysiological data. Psychophysiology from a differential perspective has been an enduring theme throughout the history of personality and temperament research. However, the present book is the first to bear the word differential in its title. Actually, this monography is not only concerned with psychophysiological personality research, but with a much broader program of systematic investigation. Multivariate research methodology permits one to operationalize physiological response profiles, both with regard to lasting differences between persons and the discrimination of situations. In order to determine functional relationships between person characteristics and situational demands, that is, to determine the processes of stimulus-response mediation, one first needs to systemize these various sources of variance in assessment models and subsequently partition the observed covariance. A series of the author's own investigations in the Hamburg and Freiburg laboratories shows just how fruitful this research approach can be.