Ionotropic glutamate receptors are ligand-gated ion channels that mediate rapid information transfer at most excitatory synapses in the brain. Crystal structures are now available for the ligand-binding domain, but the structure of the ion channel itself remains unknown. The core of the ion channel shares structural features with an inverted K(+) channel. In detail, however, differences are emerging. Most notable in the glutamate receptor ion channel are distinct structural and functional properties of a major pore-lining domain, the requirement of an additional transmembrane segment (M4), and twofold rather than fourfold symmetry. Together with ligand-binding domain structures and kinetic experiments, these findings have started to define the basic principles of channel gating in glutamate receptors.