The authors imaged electromigration-induced nanogap formation in situ by transmission electron microscopy. Real-time video recordings show that edge voids form near the cathode side. The polycrystalline gold wires narrow down until a single-grain boundary intersects the constriction along which the breaking continues. During the last 50?ms of the break, a relatively large deformation of the constriction’s geometry occurs. The shape of the anode (blunt) and the cathode (sharp) is asymmetric when the wire breaks with a bias voltage applied, but symmetric when a narrow constriction breaks spontaneously.