One of the major neuropsychological models of personality, developed by world-renowned psychologist Professor Jeffrey Gray, is based upon individual differences in reactions to punishing and rewarding stimuli. This biological theory of personality - now widely known as 'Reinforcement Sensitivity Theory' (RST) - has had a major influence on motivation, emotion and psychopathology research. In 2000, RST was substantially revised by Jeffrey Gray, together with Neil McNaughton, and this revised theory proposed three principal motivation/emotion systems: the 'Fight-Flight-Freeze System' (FFFS), the 'Behavioural Approach System' (BAS) and the 'Behavioural Inhibition System' (BIS). This is the first book to summarise the Reinforcement Sensitivity Theory of personality and bring together leading researchers in the field. It summarizes all of the pre-2000 RST research findings, explains and elaborates the implications of the 2000 theory for personality psychology and lays out the future research agenda for RST.