It has been known for half a century that neurotransmitters are released in preformed quanta, that the quanta represent transmitter-storing vesicles, and that release occurs by exocytosis. The focus of this book is twofold. In the first part, the molecular events of exocytosis are analysed. In the second part of the book, the presynaptic receptors for endogenous chemical signals are presented that make neurotransmitter release a highly regulated process.