In the human thyroid gland, the C-cell system reacts in physiologic and pathologic conditions as an entity and represents an independent endocrine organ. Its special features are its embryology, which relates it to other derivatives of the neural crest, the special histologic methods needed to demonstrate its disseminated C cells, and the obscure physiologic role of its hormone, calcitonin. Although there exist many histopathologic findings, the only C-cell disease of clinical interest is medullary carcinoma, which in its familial form is usually associated with pluriglandular syndromes.