The literature on lingual thyroid is reviewed. The incidence is relatively rare and sex incidence is four or five to one in favor of females. The embryology and development of the thyroid gland is discussed and some evidence presented to suggest that the lateral thyroid primordia are not involved in the formation of functional thyroid tissue. The pathologic findings of lingual thyroid tissue are similar to that of cervical thyroid tissue, including goiter formation. Diagnostic measures include mirror examination, I-125 scan, and tomography. Thyroid function studies are helpful in evaluating the level of thyroid function. Treatment includes surgical removal, transplantation, I-131, and thyroid replacement. Since the first three methods usually require supplemental thyroid treatment afterward, it is believed that initial thyroid replacement is the treatment of choice since it is simple, safe, effective, and noninvasive. Four new cases are reported.