Lipids are a heterogeneous group of hydrophobic or lipophilic molecules, respectively. Some of them are amphiphilic in nature, which allows them to form structures such as vesicles, liposomes, or membranes in an aqueous environment. Biochemically, lipids are made up of two subunits (or “building blocks”): ketoacyl and isoprene groups. Lipids may be classified into eight categories: fatty acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, polyketides, sterol lipids, and prenol lipids (Fig. 17.1). The main biological functions of lipids include energy storage, maintenance of the structure of cell membranes, and signaling. In the eye, lipids have numerous different functions. One example is the vitamin A-derived retinal, which is involved in the visual cycle ( Chap. 16). Another example is the prostaglandins, which are often involved in both physiological and pathophysiological processes. In this chapter, we will focus on the role of lipids in the tear film and in the retina.