Electrical impedance tomography (EIT) is a recently developed technique which enables the internal impedance of an object to be imaged non-invasively. Images are at present reconstructed from measurements made at 51 kHz with a ring of sixteen electrodes placed around the subject. A minimum data set is acquired in 40 msec, and an image can be reconstructed in about 5 sec. The technique is rapid, safe, portable and inexpensive, and so is ideal for non-invasive continuous imaging at the bedside. It cannot be used at present to image changes in the brain with scalp electrodes, as the relative resistance of the skull is too great. It should be possible to use it in the near future with a ring of subdural electrodes to produce images of brain regions undergoing anoxic depolarization in conditions such as epilepsy or stroke. It may be possible to use it in the future to image impedance changes related either to blood flow or depolarization during functional activity. Images of depolarization could be produced with a temporal resolution of milliseconds and would form a substantial advance in neuroscience methodology.