The importance of diagnostic imaging in malignant gastrointestinal tumors has moved from the primary diagnosis towards staging and follow-up studies. An accurate staging by diagnostic radiology is the cornerstone in planning radiation therapy. Especially in estimating the T- and N-stage as well as the primary diagnosis in esophagus-, gastric and rectal cancer endoscopy with endoscopic ultrasound is the method of choice. When planning radiation therapy barium studies of the esophagus and contrast enema of the colon are important. Only in the case of gastric lymphoma an upper gastrointestinal series is necessary. In the case of suspected recurrent rectal cancer CT-guided biopsy distinguishes between tumor recurrence and reactive fibrosis.