Excitatory synapses exhibit characteristic proteinaceous microcompartments at the post-synaptic membrane known as the post-synaptic density (PSD). The PSD is richly organized into multi-protein complexes composed of glutamate receptors, scaffolding proteins, kinases, cytoskeleton, and G-protein signaling effectors. The dynamic reorganizations of the higher-order architecture and composition of PSD signaling complexes underlies synaptic plasticity, learning, and memory.