Ectopic thyroid is an uncommon condition defined as the presence of thyroid tissue at a site other than the pretracheal area. When the process of embryologic migration is disturbed, aberrant thyroid tissue may appear. In most cases, ectopic thyroid is located along the embryologic descent path of migration as either a lingual thyroid or a thyroglossal duct cyst. In rare cases, aberrant migration can result in lateral ectopic thyroid tissue. Approximately 1 to 3% of all ectopic thyroids are located in the lateral neck. Ectopic tissue frequently represents the only presence of thyroid tissue; a second site of orthotopic or ectopic thyroid tissue is found in other cases. The presentation of ectopic thyroid as a lateral mass should be differentiated from metastatic thyroid cancer; other differential diagnoses include a submandibular tumor, branchial cleft cyst, carotid body tumor, and lymphadenopathy of various etiologies. In addition to the history and physical examination, the workup for a patient with a submandibular mass suspicious for ectopic thyroid should include (1) technetium-99m or iodine-131 scintigraphy, (2) ultrasonography and either computed tomography or magnetic resonance imaging, (3) fine-needle aspiration biopsy, and (4) thyroid function testing. No treatment is required for asymptomatic patients with normal thyroid function and cytology, but hypothyroid patients should be placed on thyroid hormone replacement therapy. Most cases are diagnosed postoperatively. Surgical treatment of ectopic thyroid should be considered when a malignancy is suspected or diagnosed, when the patient is symptomatic, or when thyroid suppression therapy fails.