A decade after discovery, two-P-domain potassium (K 2P) channels are recognized to be highly regulated leak pathways that control excitability, stabilizing membrane potential below firing threshold and expediting repolarization. The channels are identified by a unique structure of two pore-forming loop domains in each subunit. K 2P channels are implicated in physiological processes including anesthesia, apoptosis, neuromodulation, and sensation of oxygen tension and mechanical stress. Their study has revealed a myriad of control mechanisms in excitable cells and an underlying operational strategy: natural or pharmacological inhibition of K 2P channels promotes activity, whereas enhanced potassium leak stabilizes neurons at rest.