Although the term "thyroiditis" suggests inflammation, many disorders of the thyroid that are of unknown etiology are also currently included under this heading. Thus, only infective thyroiditis and subacute thyroiditis are clearly inflammatory; autoimmune thyroiditis is a result of lymphocytic and fibroblastic infiltration secondary to immune processes, and Riedel's struma (a rare disorder) results from a curious form of fibroblastic proliferation of unknown nature. The pathology of infective thyroiditis is that of the particular infective organism. The picture of subacute thyroiditis is that of a unique type of follicular disruption with histiocytes surrounding extravasated colloid masses, producing the appearance of giant cells. Autoimmune thyroiditis is characterized by lymphocytic infiltration, lymphoid follicles. Askanazy cells, and fibrosis, each to a variable degree, with atrophy of follicles. The fibroblastic proliferation in Riedel's struma transcends the thyroid capsule and involves adjacent tissues. The electron microscopic appearance of each form of thyroiditis is also briefly described.