General principles relevant to external beam radiation therapy (EBRT) planning of GI malignancies include simulation site-specific techniques, definition of dose-limiting structures, dose constraints, and planning principles. Specific dose prescription will be determined by tumor types and treatment approaches, for example, preoperative, definitive, or postoperative radiation therapy. The chapter presents the indications, localization, immobilization, and simulation techniques, target volumes and organs of interest definition, treatment planning and dose/fractionation for pancreatic cancer, gastric cancer, rectal cancer, and anal cancer. It also discusses the stereotactic body radiation therapy indications, special considerations at simulation, target volume definition, treatment planning and dose/fractionation for liver tumors.