Rapid eye movement sleep, the main dreaming stage, was definitively identified in 1953. However, up to 1964, the research in this field, although very significant for the first steps of knowledge, was not very extensive. In contrast, there was an explosion of results in 1965 and 1966. In this book, the author analyses all published physiological and psychological studies integrating, wherever possible, previous and more recent findings. This book is principally dedicated to research-workers entering the study of rapid eye movement sleep (REM sleep).