Psychoanalysis holds a key to the problem of sleep walking, which alone has been able to unlock the mysteries of its causes and its significance. This key is the principle of wish fulfilment, an interpretative principle which explains the mechanisms of the psyche and illuminates the mental content which underlies these. Sleep walking as a method of wish fulfilment evidently lies close to the dream life, which has become known through psychoanalysis. Most of us when we dream, according to the words of Protagoras, “lie still, and do not stir.” In some persons there is however a special tendency to motor activity, in itself a symptomatic manifestation, which necessitates the carrying out of the dream wish through walking in the sleep. The existence of this fact, together with the evidence of an influence of the shining of the moon upon this tendency to sleep walking, give rise to certain questions of importance to medical psychology. The author of this book has pursued these questions in relation to cases which have come to him for psychoanalysis, in the investigation of actual records of sleep walking given in literature and in the study of rare instances where it has been made the subject of a literary production or at least an episode in tale or drama. In each case the association with moonlight or some other light has been a distinct feature.