Medical imaging has, in the past 25 years, been revolutionized by the introduction and subsequent widespread clinical use of X-ray computerized tomography (CT). CT led the way for the introduction of several new tomographic methods that use physical techniques as diverse as positron annihilation and nuclear magnetic resonance to obtain high spatial resolution reconstructed images of both human anatomy and, more recently, human biological function. The central theme running throughout this revolution is the tomographic principle. This article describes the key elements in present-day medical imaging using tomography as the unifying concept. X-ray CT, positron emission tomography (PET) and magnetic resonance imaging (MRI) are described as far as possible in non-mathematical terms. Most space is devoted to MRI, the newest of the methods and the one which probably holds most promise for the future in terms of its range of application.