Electrical impedance tomography (EIT) is a technique which allows cross-sectional images related to the local electrical impedance within an object to be reconstructed from sets of measurements made on its surface. The main drive behind the development of EIT has been its possible application in medical imaging, as biological tissues are known to exhibit a wide range of electrical impedance and many physiological events are accompanied by electrical impedance changes. This article reviews the technical aspects of EIT as a medical imaging modality, and considers the range of applications over which it might be employed. Existing technical limitations and future developments are discussed. It is concluded that the future of EIT as a clinical diagnostic tool is likely to lie in the area of functional monitoring, where the capability of performing image-guided localized electrical impedance measurements with high acquisition speed, good sensitivity and no hazard can be exploited.