Thyroid eye disease is the most common cause of unilateral and bilateral proptosis in adults. It occurs most frequently in women aged 30 to 50 years. Clinical features include eyelid retraction, periorbital edema, conjunctival injection and chemosis, proptosis, extraocular muscle restriction, exposure keratopathy, and optic nerve compromise. Thyroid eye disease differs from idiopathic orbital myositis in that the latter is characterized by a more acute onset, more severe pain, and a rapid response to systemic corticosteroid therapy. Echography and computed tomography in thyroid eye disease reveal enlarged extraocular muscle bellies with relative sparing of the tendons. Despite evidence of an immune-mediated cause, the precise pathophysiologic mechanisms of thyroid eye disease remain unknown.