AMPA glutamate receptors mediate the majority of rapid excitatory synaptic transmission in the central nervous system1,2 and play a role in the synaptic plasticity underlying learning and memory3,4. AMPA receptors are heteromeric complexes of four homologous subunits (GluRl-4) that differentially combine to form a variety of AMPA receptor subtypes1,2. These subunits are thought to have a large extracellular amino-terminal domain, three transmembrane domains and an intracellular carboxy-terminal domain5. AMPA receptors are localized at excitatory synapses and are not found on adjacent inhibitory synapses enriched in GABAA receptors6. The targeting of neurotransmitter receptors, such as AMPA receptors, and ion channels to synapses is essential for efficient transmission7,8. A protein motif called a PDZ domain is important in the targeting of a variety of membrane proteins to cell-cell junctions including synapses8-10. Here we identify a synaptic PDZ domain-containing protein GRIP (glutamate receptor interacting protein) that specifically interacts with the C termini of AMPA receptors. GRIP is a new member of the PDZ domain-containing protein family which has seven PDZ domains and no catalytic domain. GRIP appears to serve as an adapter protein that links AMPA receptors to other proteins and may be critical for the clustering of AMPA receptors at excitatory synapses in the brain.