The 18fluorine-fluorodeoxyglucose (18F-FDG), a glucose analog, has been widely used in tumor imaging. The tumoral uptake of 18F-FDG is based upon enhanced glycolysis. Following administration, 18F-FDG is phosphorylated and trapped intracellularly that forms the basis of PET imaging. An important mechanism to transport 18F-FDG into the tumor cell is based upon the action of glucose transporter proteins; furthermore, highly active hexokinase bound to tumor mitochondria helps to trap 18F-FDG into the cell. In addition, enhanced 18F-FDG uptake may be due to relative hypoxia in tumor masses, which activates the anaerobic glycolytic pathway. In spite of these processes, 18F-FDG uptake is relatively nonspecific since all living cells need glucose. Clinical application of 18F-FDG imaging is therefore recommended in carefully selected patients.