A droopy eyelid is a common problem. Causes may range from simple and non-progressive disorders such as congenital ptosis to progressive multisystemic disorders such as mitochondrial disease. Myasthenia gravis is a very important and treatable cause of ptosis that should always be considered in the differential diagnosis. Apraxia of eyelid opening can be confused with ptosis or blepharospasm, leading to delay of the diagnosis. Horner’s syndrome has different causes and management. Knowledge of the anatomy and physiology of the eyelids is crucial for understanding the pathological processes that affect them.