Book excerpt: Conventional wisdom assumes that sleep is a resting state of the brain, with negligible activity of cortical neurons. Here, the author brings new evidence favoring the idea that during sleep, memory traces acquired while awake are consolidated. Mircea Steriade focuses on the coalescence of different sleep rhythms in interacting corticothalamic networks and on three types of paroxysmal disorders: spike-wave seizures as in absence epilepsy, Lennox-Gastaut seizures, and temporal-lobe epilepsy. Many physiological correlates of waking and sleep states as well as diverse types of epileptic seizures are also discussed.