Aponeurotic ptosis accounts for the majority of acquired ptosis encountered in clinical practice. Other types of ptosis include traumatic, mechanical, neurogenic, and myogenic. In addition to true ptosis, some patients present with pseudoptosis caused by globe dystopia, globe asymmetry, ocular misalignment, or retraction of the contralateral lid. It is particularly important for the clinician to rule out neurologic causes of ptosis such as dysfunction of the third cranial nerve, Horner's syndrome, and myasthenia gravis, as these conditions can be associated with significant systemic morbidity and mortality. A thorough history and physical examination is necessary to evaluate each patient presenting with a complaint of ptosis. Correctly identifying the cause of the patient's complaint allows the ptosis surgeon to plan for appropriate surgical repair when indicated and to defer surgery when observation or additional clinical evaluation is warranted.