The topic of this book is the pattern of relatively unadaptive movement that takes place on sudden intense stimulus, such as a loud noise (a gunshot), a sudden and unexpected painful stimulus, etc. Habituation leads to a diminution of the reaction. The normal pattern is a general flexion. The relation to certain primitive reflexes is discussed. Brain disease has little effect on the pattern. Catatonics showed exaggerated response, other psychoses no change. In epileptics the response was found to be absent. The reaction is markedly diminished under hypnosis. It is possible to abolish it by certain verbal instructions under hypnosis. The author prefers neuropathological explanations to psychological ones in trying to explain the deviations of the startle pattern. He regards it as belonging to the type of response which Goldstein has called 'catastrophic behaviour'. The book is of great interest to anybody who studies the startle reactions from the biological aspect.