Going through a phase Neuronal communication at synapses relies on regulated neurotransmitter secretion. Neurotransmitters are stored in small vesicles that are organized in clusters within nerve terminals. On stimulation, the vesicles fuse with the presynaptic plasma membrane, but despite their tight packing, replacement synaptic vesicles are rapidly recruited. Vesicles newly reformed by membrane recycling randomly intermix with the clusters. Milovanovic et al. show that synapsin, an abundant synaptic vesicle–associated protein, organizes these vesicle clusters by liquid-liquid phase separation—like oil in water (see the Perspective by Boczek and Alberti). Science , this issue p. 604 ; see also p. 548