Positron emission tomography (PET) using 18F-fluorodeoxyglucose (FDG) is considered to be a very useful adjunct to anatomic imaging techniques and is now primarily used for oncological indications. These indications include diagnosis, staging, and therapy monitoring. In this review, we discuss the articles in which FDG-PET is clinically used for monitoring therapy in breast cancer, lymphomas and gliomas. It is found that the amount of FDG uptake strongly correlates with response to therapy in breast cancer, lymphomas, and gliomas; a decrease in FDG uptake after therapy indicates a positive response to therapy. However, this conclusion is based on small patient numbers, whereas the exact response mechanism is still unknown. Therefore, more studies in comparable patient groups are required to achieve a better understanding of FDG uptake patterns after therapy. Part IIIb deals with lung, and head and neck cancer, hepatocellular and colorectal tumours, and sarcoma.