A proposal for the mechanism by which membranes, particularly vesicular and Golgi or plasma membranes, fuse during, for instance intracellular transport and secretion. The two membranes contain protein complexes, SNAREs, which will become the sites of fusion. The regretable nomenclature derives from an N-ethylmaleimide-sensitive (i.e. sulphydryl-containing) soluble fusion protein (NSF) and soluble NSF attachment proteins (SNAPs); the membrane-associated complexes that recognise SNAPS are soluble NSF attachment protein receptors (SNAREs). The vesicular membrane carries a v-SNARE and the target membrane, a t-SNARE. The two SNAREs can form a fusion particle, and with energy input from the hydrolysis of ATP, can cause membrane fusion. Spontaneous fusion is prevented by v- and t-SNARE clamping proteins.