Electrical resistance tomography (ERT) is a method that calculates the subsurface distribution of electrical resistivity from a large number of resistance measurements made from electrodes. For in-situ applications, ERT uses electrodes on the ground surface or in boreholes. It is a relatively new imaging tool in geophysics. The basic concept was first described by Lytle and Dines as a marriage of traditional electrical probing (introduced by the Schlumberger brothers) and the new data inversion methods of tomography. Development of both the theory and practice of ERT was confined mostly to the late 1980s and the 1990s. Tomographic inversion added important new capabilities as it was more general, accurate, and rigorous at spatial imaging of geophysical electrical resistance data than earlier pseudosection or curve fitting methods. An early application of geophysical ERT was to image laboratory core samples under test but practical field scale use of ERT was delayed by the lack of suitable measurement and test equipment. ERT requires the same four electrode resistance measurement used by the Schlumberger brothers (two electrodes to inject current and two other electrodes to measure the resulting potential); however, tomography requires addressing tens or hundreds of electrodes and making hundreds or thousands of such measurements in a timely fashion. Clearly, the available manual measurement systems that were designed for one, or perhaps a few measurements at a time, were not practical for ERT. High-speed, automated systems were needed.