AMPA-type glutamate receptors (AMPARs), as well as most other transmembrane proteins, are not stable in the postsynaptic density as was previously thought, but undergo constant trafficking in and out of synapses by a combination of endo/exocytosis and lateral diffusion. The respective contributions of membrane recycling events and surface trafficking to setting AMPAR numbers at synapses have been the subject of intense debate. Although this discussion is not yet settled, it is safe to state that both categories of processes participate in receptor exchange at synapses at rest and during various forms of plasticity. More unexpectedly, AMPARs can diffuse at such high rates within the postsynaptic density itself that their surface trafficking could participate not only in setting receptor numbers at individual synapses but also in tuning synaptic transmission during short-term plasticity. I here review recent results that characterize the activity-dependent properties of AMPAR surface trafficking and their possible links to fast synaptic transmission.