Continued improvements in the diagnosis and treatment of colorectal cancer are based on a clearer understanding of the pathophysiological processes underlying the disease and how it affects the patient. The addition of information derived from fluorine-18-labeled deoxyglucose (FDG) positron emission tomography (PET) scans to the cross-sectional imaging data enables a clearer understanding of the pathophysiology of the disease process. In particular, FDG PET scans are capable of demonstrating disease that mimics normal structures on conventional imaging, as well as finding disease in otherwise normal-sized lymph nodes. Abnormal areas of FDG uptake in the setting of known colorectal cancer are almost always due to recurrent disease. In addition, FDG PET offers great promise in the evaluation of treatment response, particularly as more targeted therapies become available. This report covers the basic principles underlying PET imaging, including the biochemistry of FDG and methods of PET analysis. We then, review the clinical indications for FDG PET scanning in colorectal cancer.