Contact lens usage has increased substantially world wide over the last decade so that there are currently around 100 million wearers worldwide. Apart from the conventional daily wearing schedule, extended wear and continuous wear have always attracted contact lens users. Following the launch of high permeability silicon hydrogel lenses in late 1990’s continuous wear is more and more practiced. New generation contact lenses made from silicon hydrogel and rigid gas permeable materials of high oxygen permeability provide the patients with the convenience of continuous wear as they have sufficient oxygen transmissibility to meet the oxygen requirement of the closed eye. In the open eye state cornea gets oxygen directly from the atmosphere. When the eyes are closed the oxygen supplied to the cornea is exclusively met by the capillary plexus of the palpebral conjunctiva but at much reduced levels. The normal aerobic corneal epithelial metabolism requires an adequate supply of oxygen. When the oxygen level at the anterior corneal surface is reduced, anaerobic metabolism increases and lactic acid which is the major metabolic waste product of anaerobic glycolysis accumulates in the epithelium and stroma. The increased concentration of lactate in the corneal tissue during hypoxia osmotically induces an influx of fluid into the stroma leading to corneal edema. When contact lens is worn, the oxygen supply to the cornea is met by the oxygen-rich tear lake behind the lens mediated by the lid activated tear pump and also by the diffusion of oxygen through the lens material. The tear pump is active only when the eye is open and possibly during rapid eye movement at sleep. Both soft and RGP lens materials allows diffusion of oxygen through the material but the lid activated tear pump mechanism is more effective in RGP lenses. It is estimated that a 10-20% tear exchange occurs with each blink under an RGP lens whereas only 1% tear exchange takes place under a soft lens.