The fundamental building block of neuron-to-neuron communication is the synapse, a micrometer size organelle, where the membranes of two cells come in close apposition to favor information transfer. Our deep understanding of this structure, named for the first time in 1897 by Foster and Sherrington, has evolved in parallel with the development of new technologies. Most of the main conceptual advances in our understanding of synaptic organization and function have originated from new imaging developments. Based on the new silver staining developed by Camillo Golgi, Cajal1 demonstrated that nerve cells are not continuous but contiguous, invalidating the cable theory of the nervous system. At the same time, he introduced the notion that a synapse is composed of three independent compartments: the presynapse, the postsynapse, and the space between them: the synaptic cleft. This organization remained hypothetical until the first precise image of a synapse was obtained in parallel in the 1950s by two laboratories using electron microscopy.2,3 The first image of a synapse revealed an asymmetric organization, with one compartment enriched in sized vesicle.2,4,5 This discovery and the demonstration one year later that these vesicles contained neurotransmitters,5 coupled to Katz’s electrophysiological recordings of unitary postsynaptic voltage changes, established most of the basis for our current knowledge of the mechanisms of synaptic transmission.6,7 The presynapse releases a “quantum” of neurotransmitters in the synaptic cleft due to discrete vesicle fusion, triggering a reproducible postsynaptic current. Despite the large number of newly available techniques, our present vision of the synapse is not very different from the one described by Palay, even though the invention of the patch-clamp technique offered a more robust way to measure synaptic currents8 and the revolution in genomics and proteomics allowed to allocate proteins, their interactions, and structures, into the various synaptic compartments. From the cloning of the first glutamate receptor in 19949 and the identification of PSD-95 as the main scaffold element of the postsynaptic density,10–12 to the extensive proteomic characterization of synaptic elements,13–16 it is probably safe to say that by now, most protein constituents of the synapse have been identified. However, as detailed below, we still do not fully understand how synapses work and many shadow zones remain.