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The superior tarsal muscle, also known as the Müller muscle, is an accessory smooth muscle that allows for the retraction and elevation of the upper eyelid. This muscle receives sympathetic innervation from fibers originating from the sympathetic cervical ganglion, and vascular supply from the superior branches of the ophthalmic artery. The superior tarsal muscle is often affected in thyroid eye disease-associated ophthalmopathy as the disease-mediated inflammation and fibrosis occurring within the muscle cause further upper lid retraction. Injury to the innervation of the superior tarsal muscle, as seen in Horner Syndrome, results in ptosis. The superior tarsal muscle is routinely encountered in oculoplastic surgery, including ptosis-correcting procedures. Different approaches can be used to correct ptosis, but the Müller muscle-conjunctival resection involves removing a portion of the superior tarsal muscle to remedy the ptosis (see Image. Eye—Superior Tarsal Muscle).[1][2] Understanding the anatomy and function of the superior tarsal muscle is essential in managing conditions that affect upper eyelid movement.