External beam radiation therapy (EBRT) plays a pivotal role in the management of selected cases of thyroid cancer, particularly in patients with unresectable, radioactive iodine (RAI)-refractory or aggressive/adverse histologic subtypes such as tall-cell, anaplastic and poorly differentiated thyroid carcinoma. This chapter outlines the indications, target delineation and planning techniques for EBRT, with emphasis on evidence-based guidelines and recent advancements like intensity-modulated radiation therapy (IMRT) and proton therapy. It discusses patient selection strategies, scoring systems for differentiated thyroid carcinoma and comparative insights from major international guidelines. Additionally, the chapter explores the balance between therapeutic benefit and toxicity, advocating for a multidisciplinary approach. With evolving technology and clinical experience, EBRT remains an essential adjunct in improving locoregional control and symptom palliation in thyroid cancer patients with limited treatment options.