The appearance of hybrid PET/CT scanners has made quantitative whole body scanning of radioactive tracers feasible. This paper deals with the novel concepts for assessing global organ function and disease activity based on combined functional (PET) and structural (CT or MR) imaging techniques, their advantages over current quantitative techniques and their potential clinical applications in the management of various diseases. First the complicated kinetic modeling and methods for calculation of the standardized uptake value (SUV) that have been utilized in the practice of clinical PET are briefly described. Subsequently we discuss the quantitative concepts in PET-CT imaging that have been developed in recent years: (a) SUV analysis in the dual-time point and delayed PET imaging, (b) partial volume correction of SUV for small lesions (c) assessment of global metabolic activity in the whole organ or of diseased sites and (d) the novel image segmentation techniques with FDG-PET and newer tracers to precisely define the diseased or intended normal tissue which is of great value for image guided radiation therapy.