Book excerpt: During the past several decades, a significant international research effort has been directed towards understanding the composition and regulation of the preocular tear film. This effort has been motivated by the recognition that the tear film plays a critical role in maintaining corneal and conjunctival integrity, protecting against microbial challenge and preserving visual acuity. In addition, research has been stimulated by the knowledge that alteration or deficiency of the tear film, which occurs in numerous individuals throughout the world, may lead to desiccation of the ocular surface, ulceration and perforation of the cornea, an increased incidence of infectious disease, and potentionally pronounced visual disability and blindness.