Purpose: To provide guidance on the role of adjuvant radiation therapy in the treatment of endometrial cancer. Methods: "The Role of Postoperative Radiation Therapy for Endometrial Cancer: An ASTRO Evidence-Based Guideline" by Klopp et al, published in 2014 in Practical Radiation Oncology, was reviewed for developmental rigor by methodologists. The American Society for Radiation Oncology (ASTRO) guideline content and recommendations were further reviewed by the American Society of Clinical Oncology (ASCO) Endorsement Panel. Results: The ASCO Endorsement Panel determined that the recommendations from the ASTRO guideline are clear, thorough, and based on the most relevant scientific evidence. ASCO endorsed the ASTRO guideline with several qualifying statements. Recommendations: Surveillance without adjuvant radiation therapy is a reasonable option for women without residual disease in the hysterectomy specimen and for women with grade 1 or 2 cancer and < 50% myometrial invasion, especially when no other high-risk features are present. For women with grade 1 or 2 cancer and ≥ 50% myometrial invasion or grade 3 cancer and < 50% myometrial invasion, vaginal brachytherapy is as effective as pelvic radiation therapy at preventing vaginal recurrence and is preferred. Patients with grade 3 cancer and ≥ 50% myometrial invasion or cervical stroma invasion may benefit from pelvic radiation to prevent pelvic recurrence. For women with high-risk early-stage disease and advanced disease, the ASCO Endorsement Panel added qualifying statements to the ASTRO recommendations to provide stronger statements in favor of chemotherapy (with or without radiation therapy).