In this review I discuss the evidence that some glutamatergic synapses exist that lack surface-expressed postsynaptic AMPA receptors (AMPARs) but contain NMDA receptors opposed to a functional release site. I have summarised the electrophysiological, anatomical and cell biological evidence for such postsynaptically silent synapses, and data that support the idea of rapid AMPAR insertion at silent synapses during long-term potentiation (LTP). I also discuss recent findings suggesting that developmental and activity-dependent alteration in the postsynaptic glutamate receptor composition is a general principle that occurs for other receptor subtypes. This review is not intended to provide a full discussion of possible presynaptic mechanisms for silent synapses; these are covered in the accompanying recent article [Voronin and Cherubini (this issue)].