Neurotransmitter receptors are concentrated at synaptic sites through interactions with specific scaffolding proteins and cytoskeletal elements, but can also be found in intracellular compartments or dispersed in the membrane. The notion has emerged in recent years that this distribution results from a dynamic equilibrium between the different pools of receptors. This equilibrium is regulated by neuronal activity and interactions with scaffolding proteins. A change in its set point can result in rapid variations in the number of functional receptors at synapses such as seen during plasticity. Trafficking of receptors in and out of synapses has up to now mainly been viewed within the framework of endo/exocytotic processes. After a brief presentation of the structure of excitatory and inhibitory synapses, we review data indicating that the lateral diffusion of receptors in the plane of the membrane is also a key step to regulate receptor stabilization and accumulation at synapses.