Dynamin is a protein required for vesicle formation during synaptic vesicle endocytosis (SVE) and for clathrin-mediated endocytosis. It is part of a family of large guanosine triphosphatases (GTPases), including classical dynamins, dynamin-like protein, optic atrophy 1 (OPA1), and mitofusin. Dynamin readily self-assembles into rings and helices and wraps around the necks of budding vesicles during endocytosis. It mediates membrane fission by a twisting and helical expansion action. Dynamin 1 is phosphorylated at synapses where SVE is activated by its rapid dephosphorylation. The supply of SVs is controlled by dynamin-dependent endocytosis, which is a rate-limiting step for synaptic transmission. There are several dynamin-related diseases, and dynamin inhibitors may have therapeutic utility.