Functional imaging with positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of malignant disease, image-guided therapy planning, and treatment monitoring. PET scanning with the radiolabeled glucose analogue (18)F-fluorodeoxyglucose ((18)FDG) is a relatively recent addition to the clinically available technology for imaging cancer, complementing the more conventional anatomical imaging modalities of computed tomography (CT) and magnetic resonance (MR). These modalities are complementary in the sense that CT provides accurate localization of organs and lesions while PET maps both normal and abnormal tissue function. When combined, the two modalities can identify and localize functional abnormalities. Attempts to align CT and PET data sets with fusion software are generally successful in the brain, whereas the remainder of the body is more challenging owing to the increased number of possible degrees of freedom between the two scans. Recently these challenges have been addressed by the introduction of the combined PET/CT scanner, a hardware-oriented approach to image fusion. With this device, accurately registered anatomical and functional images can be acquired for each patient in a single scanning session. Currently, over 400 combined PET/CT scanners are installed in medical institutions worldwide, almost all of them for the diagnosis and staging of malignant disease. However, the real impact of this technology undoubtedly will be for cancer therapy, where PET/CT images will be used to guide biopsies and assist in surgical intervention, to define target volumes for radiation therapy and optimize dose, and to monitor response to chemotherapy and establish individualized patient treatment strategies.