Positron Emission Tomography/Computerized Emission Tomography (PET/CT) creates fusion images which are a combination of tissue function (PET) and anatomy (CT). PET/CT imaging for esophageal and colorectal cancers improves staging, detection of recurrence, and treatment monitoring. Both tumor types are highly metabolically active in the untreated state, and the glucose analog, 2-[18F]-2-fluoro-2-deoxy-D-glcuose, (FDG), is widely useful as a PET imaging tracer for these malignancies. For esophageal and colorectal malignancies, diagnostic imaging sensitivity and specificity are greater than 90%. For esophageal tumors, PET imaging is standard of care for staging of locally advanced tumor. Fifteen to 20% of patients will have other wise occult metastatic sites detected by PET imaging at initial staging. Progression free survival and survival are correlated with the magnitude of reduction in PET-FDG measured metabolic activity post-treatment. For colorectal cancers, PET-FDG is standard of care to detect recurrence, and to monitor treatment response. The addition of CT to the PET imaging, causes a significant reduction in uncertainty of image interpretation and improves the distinction between benign and malignant causes for FDG-PET positive sites.