Abstract: ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET or PET-CT is an accurate test for differentiating residual viable tumor tissue from therapy-induced changes in tumor. Furthermore, quantitative assessment of therapy-induced changes in tumor ¹⁸F-FDG uptake may allow the prediction of tumor response. Treatment may be adjusted according to tumor response. So it is increasingly used to monitor tumor response in patients undergoing chemotherapy and chemoradiotherapy. Here we focused on practical aspects of ¹⁸F-FDG PET or PET-CT for treatment monitoring and on the existing advantages and challenges.