As in the case of many ligand-gated ion channels, the biochemical and electrophysiological properties of the ionotropic glutamate receptors have been studied extensively. Nevertheless, we still do not understand the molecular mechanisms that harness the free energy of agonist binding, first to drive channel opening, and then to allow the channel to close (desensitize) even though agonist remains bound. Recent crystallographic analyses of the ligand-binding domains of these receptors have identified conformational changes associated with agonist binding, yielding a working hypothesis of channel function. This opens the way to determining how the domains and subunits are assembled into an oligomeric channel, how the domains are connected, how the channel is formed, and where it is located relative to the ligand-binding domains, all of which govern the processes of channel activation and desensitization.