Positron emission tomography (PET) using (18)F-fluorodeoxyglucose (FDG) is increasingly used in the diagnostic management of colorectal cancer patients. It provides a highly sensitive and specific diagnosis which is entirely based upon alterations of the glucose metabolism found in malignant tissues. The information provided by FDG-PET is independent of the underlying structural characteristics of the lesions and, therefore, it is essentially complementary to the available structural imaging modalities such as CT, MRI and (endoscopic) ultrasound. Several studies have now been performed on the use of FDG-PET in colorectal adenocarcinoma for primary pre-operative staging, for diagnosis and (re)staging of recurrent disease, for localization and staging of occult recurrent disease, and for the assessment of the metabolic effects of chemotherapy and radiotherapy. This chapter aims to clarify some fundamental issues of both detection device and radiotracer, the proven indications for FDG-PET, the strength and limitations of the technique, and how its implementation would affect patient management.