Positron emission tomography (PET) is a novel method in nuclear medicine to image metabolism in vivo. A wide range of physiological tracers labelled with positron emitters are available for dynamic measurements of various physiological and pathological functions of the body. PET is nowadays widely used for cardiological and neurological studies in health and disease. Oncological applications are under eager investigations. In particular, the detection of viable residual and recurrent tumour is a difficult clinical challenge which is expected to be solved by PET, while the altered metabolism related to cancer can also be studied.