There has recently been an increasing interest in the possibility of producing images of electrical impedance within the human body. When an electric current is applied to the body of a voltage distribution is developed across the body surface. This distribution is in part dependent on the internal impedance distribution within the body and it is possible to estimate this distribution from a suitable set of voltage measurements. Because of the nonlinear relationship between the impedance distribution and the voltage distribution at the surface of the body, the reconstruction problem is much more difficult than for other tomographic imaging techniques, but a significant amount of progress has been made, and it is now possible to produce tomographic images of in vivo distributions of impedance, albeit with low spatial resolution. Future developments should improve image quality.