Initial research on individual differences in personality centered on the development of questionnaires. These were the only available tools for the description and prediction of behavior. Later, trait models were developed that attempted to move from personality description to a multilevel causal form of explanation. As noted by Gale & Eysenck (1992), psychometric methods have achieved good levels of predictive validity for psychologically and socially important human behaviors. This success in describing personality bolstered attempts to identify the biological substrate of psychometric derived indices of individual variation. Scores on psychometric instruments are examined using a variety of procedures that record bioelectrical signals. These include procedures to monitor autonomic nervous system activity, such as electrodermal and cardiac recordings, and cortical activity, with electroencephalographic and event-related potential procedures. Investigations of elementary physiological events in normal thinking, feeling, and interacting individuals are now feasible. The techniques provide windows through which psychological processes and neurological generators of bioelectric activity can be observed unobtrusively (see, e.g. the recent Handbook of Psychophysiology by Cacioppo et al. 2000). A number of different psychophysiological responses and methods are applied to verify hypotheses and for further exploration of the biological bases of temperament and personality dimensions, notably extraversion (E), neuroticism (N) and sensation seeking (SS). The research on biological and psychophysiological determinants of temperament and personality emerged from specific hypotheses proposed by Pavlov and Eysenck in their classic experimental research on personality (Eysenck 1963, 1967; Nebylitsyn & Gray 1972; Strelau 1983). This chapter begins with a brief outline of the main hypotheses that were explored using psychophysiological methods to study individual differences in personality, with specific emphasis on the extraversion trait. Subsequently, the most consistent psychophysiological results will be described and discussed.