During the twentieth century, only two researchers published world famous, encyclopedic monographs related to sleep. The first of these works appeared in 1913, when Henri Pieron published Le probleme physiologique du sommeil , which is still a standard reference today. Although multiple researchers and clinicians, such as Economo (1917, 1928), Berger (1929), Hess (1931), Bremer (1935, 1936), Loomis et al. (1937) and Ranson (1939), subsequently published highly important findings on sleep-waking processes, the next fundamental phase was marked by the publication of the first edition of Sleep and Wakefulness, by Nathaniel Kleitman in 1939. Later, following his discovery of rapid eye movement (REM) dreaming sleep stage in 1953, Kleitman published the masterful second edition of this work in 1963. Later on, several researchers published highly important reviews (Jouvet 1962, 1972, Moruzzi 1963, 1972, Hobson 1988, Steriade and McCarley 1990), although they were directed to more limited topics. The book provides an overview of Pieron’s and Kleitman’s sleep-related papers, placed in their proper historical context.