This document discusses the differential diagnosis and approach to evaluating red eye. It begins by listing conjunctivitis, scleritis, ocular foreign body, and angle closure glaucoma as possible causes of red eye. For acute onset painful red eye, common unilateral causes include corneal abrasion, foreign body, and trauma, while bilateral causes include chemical injury, contact lenses, dry eyes, and allergies. Acute painless unilateral red eye may be viral conjunctivitis or subconjunctival hemorrhage, while bilateral causes are blepharitis and allergies. Chronic red eye may result from blepharitis, dry eye, conjunctivitis, contact lenses, or thyroid eye disease.