This book presents for the first time a view of a sensory system working in a different state - that of the sleeping brain. The auditory system is always open receiving information from the environment and the body itself (conscious and unconscious data). Even during sleep the auditory information is processed, although in a different way. This book draws information from evoked potentials, FMRI, PET, SPECT, lesions, etc., together with electrophysiological online data in order to depict how the auditory system single unit activity, recorded during sleep, revealed the possibility of sensory information participation in sleep processes.This book presents diverse experimental viewpoints from the beginning of classical electroencephalography to the more recent imaging, single units, electro-magneto-encephalography studies, etc. It includes classic data as well as new data based in the existing literature and on the long scientific research lines (auditory and sleep) developed by the author and co-workers on this subject since 1963.