Neurotransmitter release from presynaptic nerve endings is mediated by Ca2+-dependent exocytosis of synaptic vesicles. During the past 15 years, major progress has been made in unravelling the molecular mechanisms underlying exocytosis and the recycling of synaptic vesicles. Exocytotic membrane fusion is mediated by the SNARE proteins synaptobrevin/VAMP, syntaxin 1, and SNAP-25, which are the only substrates of the Tetanus and Botulinum neurotoxin proteases. Upon membrane contact, the vesicular SNARE synaptobrevin forms complexes with the plasma membrane-resident SNAREs SNAP-25 and syntaxin 1. Complex formation proceeds from the N-terminal end towards the C-terminal membrane anchors, thus pulling the membranes together and initiating fusion (“zipper” hypothesis of SNARE function). In our own work, we have focussed on understanding the mechanisms of SNARE zippering in more detail using biochemical approaches and in-vitro fusion reactions with native and artificial membranes. Furthermore, we have studied the role of endosomes in synaptic vesicles and have performed a quantitative analysis of purified synaptic vesicles. The resulting molecular model yields novel insights into the structure of biological membranes, sheds light on neurotransmitter uptake and storage by synaptic vesicles, and provides a starting point for quantitative molecular models of vesicle docking and fusion.