In the developed world, images of brain structure are available as an everyday diagstic aid, and the characteristic appearances of most pathological conditions can be looked up in a textbook. Functional brain imaging is to this day less widely used, partly because most pressing diagstic questions can be answered by refer- ence to the patient's cerebral anatomy, partly for reasons of technical limitations of functional techniques. PET as a technique is sufficiently resource-demanding and complex to inhibit its use as an everyday diagstic technique. SPECT lacked suitable tracers for many years, and early systems had poor spatial resolution. However, rotating gamma camera techlogy has advanced to the point where images of the brain of reasonable quality can be obtained at most large hospitals, and practical tracers, particularly of regional cerebral blood flow, are easily avail- able. As research advances, clinical applications are emerging. A recent report of the Therapeutics and Techlogy Assessment Subcommittee of the American Academy of Neurology! details a number of currently recognised clinical appli- cations, some of which are dealt with in this book. Given this recognition, it is increasingly important that clinicians (particularly neuroclinicians, psychiatrists and specialists in cerebrovascular disease), nuclear medicine specialists and physicists acquire an idea of the major applications of the technique, and the research background on which these applications are based.