This chapter discusses the nature of neurophysiological processes and the utility of physiological measures as state-of-the-art empirical indexes of constructs fundamental to social psychological theories. It covers the relevant background information, including the evolution of social psychophysiology. The chapter provides a brief discussion of relevant epistemological issues, and the nature of physiological indexing. It briefly reviews and integrates general information regarding physiological control processes and general technological approaches to their measurement. The chapter reviews information important to psychophysiological indexing. It also reviews the rationale underlying the index and its validation and provides an example or two of its use. The chapter also discusses the threats to validity in physiological measurement. It presents illustrations of state-of-the-art physiological indexes of important motivational and affective constructs. Specific psychophysiological indexes derive their validity from psychophysiological theory confirmed via systematic empirical work.