In the two last years my lectures were addressed to the biological bases of wakefulness and REM sleep. The present lecture is focused on slow wave sleep, which corresponds to stages 3 and 4 in humans. These two sleep stages both present high voltage slow waves in the EEG that belong to the delta band (less than 3 Hz). Slow wave sleep usually lasts between 70 and 90 minutes in normal individuals, and takes place during the first hours of the night, as though our organism was making sure we got this essential sleep. Slow wave sleep is: a) the first sleep stage to recuperate after sleep deprivation, b) the stage with the most constant length even in individuals who have very different sleep habits, and c) the sleep stage that remains constant during experiments of gradual sleep reduction for eight-hour sleepers. This sleep stage is the one in which growth hormone is released and it essential for physical and intellectual efficiency and normal behavior in young adult individuals. Slow wave sleep is remarkably reduced in old people. What brain structures are responsible for the bioelectric and behavioral events of the slow waves sleep? Both the cerebral cortex and thalamus are indispensable for the existence of the slow waves EEG characteristic to this stage. Other brain structures participate in slow wave sleep organization; these include the basal forebrain, the oral pontine reticular nucleus, the deep cerebellar nuclei and the solitary tract nucleus. In this presentation I consider the most relevant functions of these structures as well as their anatomical connections, mainly with the neuronal networks responsible for other stages of the sleep-wakefulness cycle.