The cornea is the anterior, transparent part of the collagenous wall of the eyeball. It is the window of the eye to the outer world. Its properties allow for the formation of an optical image on the light-sensitive retina in the back of the eye. This requires transparency and regularity, but it also demands that the gross dimensions of the eye be kept constant. A regulated hydrostatic pressure within a relatively stiff eyeball accomplishes this. The aim of this chapter is to describe the human cornea. Regarding dimensions, there are of course large species variations. Functional aspects have often been studied in animal corneas and usually extended to all other species. Unless otherwise stated the text applies to the "standard human cornea".