In the nervous system, the intracellular chloride concentration ([Cl(-)](i)) determines the strength and polarity of gamma-aminobutyric acid (GABA)-mediated neurotransmission. [Cl(-)](i) is determined, in part, by the activities of the SLC12 cation-chloride cotransporters (CCCs). These transporters include the Na-K-2Cl cotransporter NKCC1, which mediates chloride influx, and various K-Cl cotransporters--such as KCC2 and KCC3-that extrude chloride. A precise balance between NKCC1 and KCC2 activity is necessary for inhibitory GABAergic signaling in the adult CNS, and for excitatory GABAergic signaling in the developing CNS and the adult PNS. Altered chloride homeostasis, resulting from mutation or dysfunction of NKCC1 and/or KCC2, causes neuronal hypoexcitability or hyperexcitability; such derangements have been implicated in the pathogenesis of seizures and neuropathic pain. [Cl(-)](i) is also regulated to maintain normal cell volume. Dysfunction of NKCC1 or of swelling-activated K-Cl cotransporters has been implicated in the damaging secondary effects of cerebral edema after ischemic and traumatic brain injury, as well as in swelling-related neurodegeneration. CCCs represent attractive therapeutic targets in neurological disorders the pathogenesis of which involves deranged cellular chloride homoestasis.