ClC channels constitute a large family of Cl(-)-selective ion channels that are present in diverse organisms. The channels have a complex gating behavior, in which channel opening and closing is tightly coupled to ion permeation. Recent success in the structure determination of bacterial channels has revealed the overall architecture of this family and provided important insight into ion selectivity and gating. The ClC channels are homodimers, with each subunit containing an ion conduction pore. In the narrow selectivity filter of each pore, Cl(-) are bound to several sites through electrostatic interactions with helix dipoles and the partial charges of protein residues. In the closed conformation of the channel, a conserved glutamate residue occupies one of the ion-binding sites; on channel opening, this glutamate residue moves out of the binding site, thereby making room for an additional Cl(-) ion.