The state of affairs in the field of sleep deprivation has been discussed in this chapter. Special emphasis was placed on total sleep deprivation, a domain in which the most important work has been done since the dawning of this century, and especially during the 'golden years' between 1955 and 1975. After a discussion of the biorhythmical aspects, the general condition of sleep deprived subjects was pointed out. Psychological-psychiatric changes and impairment of performance were further topics of discussion, followed by sections on EEG, neurological, autonomic and metabolic changes. The relationship of SD to epileptic phenomena and endogenous depression has been considered. The characteristics of recovery sleep have also been presented. Most of the author's impressions are based on personal data, and especially on 2 subjects who underwent most elaborate testing. A discussion of partial sleep deprivation and selective sleep deprivation is also included.