Most pathological conditions that arise within the human body are the result of underlying biochemical defects, and often the success or failure of subseq-uent treatment depends upon an early identification of these defects. For this reason, medical diagnostic techniques that reflect biochemical or metabolic behaviour have a special significance. One such technique, positron emission to-mography (PET), is based on the radiolabeling of substrates that are involved in the fundamental biochemical processes of the body. External detection and imaging of the radiation emitted by the radiolabel enables the chosen substrate to be monitored as it progresses along a particular metabolic pathway. In as far as the image of the emitted radiation reflects the biochemistry of the substrate in the body, abnormalities of biochemistry may be identified from the image. Thus, apart from the introduction into the body of a small quantity of radioac-tivity, PET is entirely non-invasive and without undue risk to the patient. To be successful, such an approach will require: the development of suitable instrumentation for imaging the radiation, the development of radiolabels with a well-understood fate within the body, the development of mathematical models that will explain in simple terms the kinetic behaviour of the tracers so as to distinguish pathology from normal function.