Paradoxical sleep (PS) is characterized by EEG activation with a disappearance of muscle tone and the occurrence of rapid eye movements (REM) in contrast to slow-wave sleep (SWS, also known as non-REM sleep) identified by the presence of delta waves. Soon after the discovery of PS, it was demonstrated that the structures necessary and sufficient for its genesis are restricted to the brainstem. We review here recent results indicating that brainstem glutamatergic and GABAergic, rather than cholinergic and monoaminergic, neurons play a key role in the genesis of PS. We hypothesize that the entrance to PS from SWS is due to the activation of PS-on glutamatergic neurons localized in the pontine sublaterodorsal tegmental nucleus. The activation of these neurons would be due to a permanent glutamatergic input arising from the lateral and ventrolateral periaqueductal gray (vlPAG) and the removal at the onset of PS of a GABAergic inhibition present during W and SWS. Such inhibition would be coming from PS-off GABAergic neurons localized in the vlPAG and the adjacent deep mesencephalic reticular nucleus. The cessation of activity of these PS-off GABAergic neurons at the onset and during PS would be due to direct projections from intermingled GABAergic PS-on neurons. Activation of PS would depend on the reciprocal interactions between the GABAergic PS-on and PS-off neurons, intrinsic cellular and molecular events, and integration of multiple physiological parameters.