Riedel’s thyroiditis is a chronic sclerosing (fibrosing) replacement of the gland that is exceedingly rare. The process extends to adjacent structures, making any surgical intervention very difficult and potentially harmful. The exact cause of Riedel’s thyroiditis remains unknown, though an autoimmune relation is suspected, due to elevated titers of thyroid auto-antibodies. It is generally considered as a part of a multifocal fibro-inflammatory process also involving other tissues such as the mediastinum, liver, lung, retro-peritoneum, and orbit. No specific treatment is available beyond limited resection of the thyroid gland to relieve the symptoms of tracheal or esophageal compression. Riedel’s thyroiditis should be suspected in patients presenting with a thyroid mass and unique clinical features. Findings that increase the likelihood of Riedel’s thyroiditis include local restrictive or infiltrative symptoms out of proportion to the size or extent of the mass or simultaneous hypocalcemia. Surgical intervention should be limited to rule out the presence of malignancy and obtain the histopathological confirmation of the disease. Once the diagnosis of Riedel’s thyroiditis is established, a search for related fibrotic conditions and medical treatment should be pursued. Replacement with levothyroxine (L-T4) and, when appropriate, calcium and active vitamin D metabolites should be started when indicated, along with anti-inflammatory medications. This chapter discusses the epidemiology, pathology, etiology, clinical presentation, evaluation, and management of Riedel’s thyroiditis.