PET with the glucose analog (18)F-FDG is increasingly used to monitor tumor response in patients undergoing chemotherapy and chemoradiotherapy. Numerous studies have shown that (18)F-FDG PET is an accurate test for differentiating residual viable tumor tissue from therapy-induced fibrosis. Furthermore, quantitative assessment of therapy-induced changes in tumor (18)F-FDG uptake may allow the prediction of tumor response and patient outcome very early in the course of therapy. Treatment may be adjusted according to the chemosensitivity and radiosensitivity of the tumor tissue in an individual patient. Thus, (18)F-FDG PET has an enormous potential to reduce the side effects and costs of ineffective therapy. This review focuses on the practical aspects of (18)F-FDG PET for treatment monitoring and on how to perform a quantitative assessment of tumor (18)F-FDG uptake in clinical studies.