In the twenty five years since my previous review was published in the Ergebnisse der Physiologie (Eccles 1936), the subject of synaptic transmission has been transformed. The enormous advances in knowledge and understanding have been brought about largely as a result of the microtechniques: electron-microscopy; electrical investigations by microelectrodes, particularly with intracellular electrodes; and micro-injection procedures, both extracellular and intracellular. There is an excellent review of synaptic transmission immediately before the introduction of these microtechniques (Fessard And Posternak 1950). Relatively few types of synaptic junctions have yet been subjected to intensive investigation by these new procedures; nevertheless one can have confidence that general principles of synaptic action are being established because there has been a remarkable uniformity of the essential features of synaptic actions for a wide variety of junctional regions in invertebrates as well as vertebrates.