Transcranial electrical stimulation (tES) is a neuromodulatory technique in which low voltage constant or alternating currents are applied to the human brain via scalp electrodes. The basic idea of tES is that the application of weak currents can interact with neural processing, modify plasticity and entrain brain networks, and that this in turn can modify behaviour. The technique is now widely employed in basic and translational research, and increasingly is also used privately in sport, the military and recreation. The proposed capacity to augment recovery of brain function, by promoting learning and facilitating plasticity, has motivated a burgeoning number of clinical trials in a wide range of disorders of the nervous system.