Tear film physiology is currently based on the anatomy of each of the 3 layers, on the changes occurring with blinking and on the physicochemical properties of the tear fluid. Abnormalities in tear film may be due to deficiency of aqueous tears or mucin, changes in the lipid layer, inadequate blinking or epithelial pathology. Patients with marginal deficiencies are usually symptom-free but will develop symptoms promptly if contact lenses are fitted.