The lid margin is an underestimated structure for preservation of ocular surface health and development of dry eye disease.

Purpose: The structure of the lid margin is insufficiently understood and defined, although it is of obvious importance in ocular surface integrity.

Methods: The structure and function of the different zones of the lid margin are explained with a focus on dry eye disease.

Results: The posterior lid margin, which is of particular significance for the integrity of the ocular surface, includes the meibomian glands that open within the cornified epidermis. Their obstructive dysfunction is a main cause of dry eye disease. The orifice is followed by the mucocutaneous junction, which extends from the abrupt termination of the epidermis to the crest of the inner lid border. The physiological vital stainable line of Marx represents its surface, and can be used e.g. as a diagnostic tool for the location and functionality of the meibomian gland orifices and lacrimal puncta. The marginal conjunctiva starts at the crest of the inner lid border and forms a thickened epithelial cushion. This is the point closest to the globe, and represents the zone that wipes the bulbar surface and distributes the thin preocular tear film. It is hence termed the 'lid wiper' and pathological alterations that result in a vital staining are a sensitive early indicator of dry eye disease.

Conclusions: The margin of the eyelid is an important but currently underestimated structure in the maintenance of the preocular tear film and of the utmost importance for the preservation of ocular surface integrity and in the development of dry eye disease.