Recent work has helped reconcile puzzling results from brainstem transection studies first performed over 60 years ago, which suggested the existence of a sleep-promoting system in the medullary brainstem. It was specifically shown that GABAergic neurons located in the medullary brainstem parafacial zone (PZGABA) are not only necessary for normal slow-wave-sleep (SWS) but that their selective activation is sufficient to induce SWS in behaving animals. In this review we discuss early experimental findings that inspired the hypothesis that the caudal brainstem contained SWS-promoting circuitry. We then describe the discovery of the SWS-promoting PZGABA and discuss future experimental priorities.