The differential diagnosis of eyelid erythema and edema is broad, ranging from benign, self-limiting dermatoses to malignant tumors and vision-threatening infections. A definitive diagnosis usually can be made on physical examination of the eyelid and a careful evaluation of symptoms and exposures. The finding of a swollen red eyelid often signals cellulitis. Orbital cellulitis is a severe infection presenting with proptosis and ophthalmoplegia; it requires hospitalization and intravenous antibiotics to prevent vision loss. Less serious conditions, such as contact dermatitis, atopic dermatitis, and blepharitis, are more common causes of eyelid erythema and edema. These less serious conditions can often be managed with topical corticosteroids and proper eyelid hygiene. They are differentiated on the basis of such clinical clues as time course, presence or absence of irritative symptoms, scaling, and other skin findings. Discrete lid lesions are also important diagnostic indicators. The finding of vesicles, erosions, or crusting may signal a herpes infection. Benign, self-limited eyelid nodules such as hordeola and chalazia often respond to warm compresses, whereas malignancies require surgical excision.