The synapse is a specialized junctional complex by which axons and dendrites emerging from different neurons intercommunicate [12]. This was proposed first by Sherrington in 1897, who also proposed the term synapse. The existence of synapses was immediately demonstrable by EM and can be recognized today in a dynamic fashion by Nomarski and confocal optics, light microscopy and scanning EM. With the development of neurochemical approaches to neurobiology, an understanding of synaptic form and function becomes of fundamental importance. As was noted in the first ultrastructural study on synapses (Palade and Palay in 1954, quoted in [4]), synapses display interface specialization and frequently are polarized or asymmetrical. The asymmetry is due to the unequal distribution of electron-dense material, or thickening, applied to the apposing membranes of the junctional complex and the heavier accumulation of organelles within the presynaptic component. The closely applied membranes constituting the synaptic site are overlaid on the presynaptic and postsynaptic aspects by an electron-dense material similar to that seen in desmosomes and separated by a gap or cleft of 15 to 20 nm. The presynaptic component usually contains a collection of clear, 40- to 50-nm synaptic vesicles. These synaptic vesicles are important in packaging, transport and release of neurotransmitters and after their discharge into the synaptic cleft, they are recycled with theaxon terminal [6,13]. Also present are small mitochondria approximately 0.2 to 0.5 μm in diameter (Figs. 1-8–1-10). Occasionally, 24-nm microtubules, coated vesicles and cisternae of the smooth ER are found in this region. On the postsynaptic side is a density referred to as the subsynaptic web, but apart from an infrequent, closely applied packet of smooth ER or subsurface cisternae belonging to the hypolemmal system, there are no aggregations of organelles in the dendrite. At the neuromuscular junction, the morphological organization is somewhat different. Here, the axonterminal is greatly enlarged and ensheathed by Schwann cells; the postsynaptic or sarcolemmal membrane displays less density and is infolded extensively.