Radiation therapy has recently emerged as a pivotal modality in the management of completely resected, high-risk gastric cancer. The recently published results of the Intergroup 0116 Gastric Surgical Adjuvant Trial randomized high-risk (T3,4 and/or node positive), completely resected gastric or gastroesophageal adenocarcinomas to receive either observation alone or radiochemotherapy after complete resection. Radiochemotherapy produced significant improvements in relapse-free (p < 0.0001) and overall survival (p = 0.01). Radiation oncologists must now clearly comprehend the principles governing the rationale supporting this therapy to apply it to those afflicted with this disease. This paper represents a consensus report reviewing data supporting radiotherapy, important clinical and anatomic issues related to radiotherapy, and details of the practical application of radiation therapy to commonly occurring clinical presentations. Supportive therapy during and after radiochemotherapy is also discussed.