How do human emotions arise, what functions do they serve, what is their evolutionary background, how do they relate to behaviour and the brain? These questions are put, and answered, in relation to the emotion of fear in this, the second edition of professor Gray's extremely well known book, first published in 1971. In this edition, the text has been extensively modified and brought up-to-date, but the book maintains the style and general argument of the first edition. The author's approach in this book is from a biological standpoint; he emphasises the evidence that has accumulated from experiments by psychologists, ethologists, physiologists and endocrinologists. Although a lot of this evidence has been obtained from animal studies, it throws light on the psychology and physiology of fear in Man. Differences between individuals in their susceptibility to fear are treated with as much attention as the common factors are.