Purpose: The American Society for Radiation Oncology (ASTRO) produced an evidence-based guideline on external-beam radiotherapy for patients with locally advanced non-small-cell lung cancer (NSCLC). Because of its relevance to the American Society of Clinical Oncology (ASCO) membership, ASCO endorsed the guideline after applying a set of procedures and a policy that are used to critically examine and endorse guidelines developed by other guideline development organizations. Methods: The ASTRO guideline was reviewed by ASCO content experts for clinical accuracy and by ASCO methodologists for developmental rigor. On favorable review, an ASCO expert panel was convened and endorsed the guideline. The ASCO guideline approval body, the Clinical Practice Guideline Committee, approved the final endorsement. Results: The recommendations from the ASTRO guideline, published in Practical Radiation Oncology, are clear, thorough, and based on the most relevant scientific evidence. The ASCO Endorsement Panel endorsed the guideline and added qualifying statements. Recommendations: For curative-intent treatment of locally advanced NSCLC, concurrent chemoradiotherapy improves local control and overall survival compared with sequential chemotherapy followed by radiation. The standard dose-fractionation of radiation is 60 Gy given in 2-Gy once-daily fractions over 6 weeks. There is no role for the routine use of induction therapy before chemoradiotherapy. Current data fail to support a clear role for consolidation therapy after chemoradiotherapy; however, consolidation therapy remains an option for patients who did not receive full systemic chemotherapy doses during radiotherapy. Important questions remain about the ideal concurrent chemotherapy regimen and optimal management of patients with resectable stage III disease.