Membrane fusion processes, mediated by the SNARE proteins, are a hallmark of eukaryotic life. We are especially interested in membrane fusion during neuronal exocytosis and in endosomal fusion processes. A number of in vitro fusion assays with these proteins reconstituted in artificial membranes have been established over recent years. However, it has still been proven difficult to monitor intermediate stages of the fusion process in a system that captures the essential features of the in vivo system. We develop and apply a reconstituted membrane system based on pore-spanning membranes allowing for a quantitative analysis of the different stages during the fusion of a single vesicle, such as docking, hemifusion and full fusion.