Concentration of neurotransmitter receptors at synapses is thought to result from stable binding to subsynaptic scaffold proteins. Recent data on synaptic plasticity have shown that changes in synaptic strength derive partly from modification of postsynaptic receptor numbers. This has led to the notion of receptor trafficking into and out of synapses. The proposed underlying mechanisms have under-evaluated the role of extrasynaptic receptors. Recent technological advances have allowed imaging of receptor movements at the single-molecule level, and these experiments demonstrate that receptors switch at unexpected rates between extrasynaptic and synaptic localizations by lateral diffusion. Variation in receptor numbers at postsynaptic sites is therefore likely to depend on regulation of diffusion by modification of the structure of the membrane and/or by transient interactions with scaffolding proteins. This review is part of the TINS Synaptic Connectivity series.