Topical administration to the eye is usually characterised by very poor drug bioavailability due to several natural defence mechanisms, including nasolacrymal drainage, blinking, and poor permeability of the cornea. Enhancement of the corneal permeability to drug molecules is one of the strategies to improve the efficiency of topical drug delivery to the eye. Several classes of compounds have been researched as potential penetration enhancers through ocular membranes. These include chelating agents, cyclodextrins, surfactants, bile acids and salts, and crown ethers. There are also reports on the use of cell penetrating peptides and chitosan as penetration enhancers in ocular drug delivery. The most commonly used penetration enhancers in ocular formulations are benzalkonium chloride and ethylenediamine tetraacetate (EDTA). Benzalkonium chloride is often used as an antimicrobial preservative in eye drops and EDTA is used as a chelating agent.