The three major aims of imaging in esophageal and gastric cancer are to distinguish between locoregional and systemic disease (M stage), to determine local tumor extension (T and N stages), and to assess response to chemotherapy or chemoradiotherapy. Depending on the applied standard of reference, the sensitivity of computed tomography (CT) for detection of distant metastases ranges between less than 50% to greater than 90%. In esophageal cancer, positron emission tomography with the glucose analog fluorodeoxyglucose (FDG-PET) has been shown to detect metastatic disease in approximately 20% of the patients who were considered to have only locoregional disease with CT. In clinical studies, endoscopic ultrasound (EUS) has been shown to differentiate between tumor stages T1/T2 and stages T3/T4 with an accuracy of more than 90%. Accuracy of EUS for differentiating individual tumor stages in routine clinical use has been reported to be markedly lower. Assessment of tumor response by FDG-PET has been shown to correlate with histopathologic tumor regression and patient survival. Furthermore, quantitative measurements of tumor FDG uptake may predict histopathologic tumor response and patient outcome as early as 2 weeks after initiation of preoperative chemotherapy.