Positron emission tomography using the glucose analog fluorine-18 fluorodeoxyglucose (FDG-PET) provides a unique means of non-invasive assessment of tumor metabolism. Several approaches, of varying complexity, can be applied for quantitative image analysis. Previous studies have demonstrated that "standardized uptake values" (SUV) and simplified tracer kinetic modeling, using the "Patlak-Gjedde"-analysis, provide highly reproducible parameters of tumor glucose utilization. Quantification of regional FDG uptake gives complementary information to visual image interpretation and provides objective criteria for differentiation between benign and malignant lesions. Moreover, quantification of tumor glucose metabolism is essential for assessment of therapy induced changes. Clinical studies in breast cancer and lymphoma suggest that serial FDG-PET studies allow the prediction of response early in the course of chemotherapy. Therefore, FDG-PET may be helpful in patient management by avoiding ineffective chemotherapy and supporting the decision to continue dose intense regimes. In addition, FDG-PET allows non-invasive assessment of tumor viability following chemo- and radiotherapy which permits individualized therapy management.