**Abstract**

The precorneal tear film is a thin layer, about 2–5.5 μm thick, which overlays the corneal and conjunctival epithelium. It functions to lubricate and protect the corneal and eyelid interface from environmental and immunological factors as well as provide an optical medium. The tear film is depicted as a three-layered structure: lipid, aqueous, and mucous layers. Within each layer possesses a different composition which dictates its function. In common between the three layers are their homeostatic process of evaporation and drainage. Any dysfunction in either of the layers can result in Dry Eye Syndrome (DES). The composition, regulation, and pathology of tear film will be discussed in this chapter.

**Keywords:** Physiology, tear film, meibomian glands, lacriminal glands, conjunctival goblet cells, blink reflex
