Lumican, a prototypic leucine-rich proteoglycan with keratan sulfate side chains, is a major component of the cornea, dermal, and muscle connective tissues. In these bifunctional molecules, the protein moiety binds collagen fibrils and the highly charged hydrophilic glycosaminoglycans regulate interfibrillar spacings. Lumican is the major keratan sulfate proteoglycan of the cornea but is also distributed in interstitial collagenous matrices throughout the body. Lumican regulates collagenous matrix assembly as a keratan sulfate proteoglycan in the cornea and is also present in the connective tissues of other organs and embryonic corneal stroma as a glycoprotein. Lumican may regulate collagen fibril organization and circumferential growth, corneal transparency, and epithelial cell migration, and tissue repair. lumican expressed in injured epithelium may modulate cell behavior such as adhesion or migration, thus contributing to corneal epithelial wound healing. Lumican plays a crucial role in the regulation of collagen assembly into fibrils in various connective tissues and serve as a definitive link between a necessity for lumican in the development of a highly organized collagenous matrix and corneal transparency.