The thinning of the cornea that occurs in keratoconus has been well described; however, the mechanism of tissue degradation remains unknown. Elevated proteinase activity is one possibility and approximately 20 publications over the last 20 years have addressed this hypothesis. Early studies reported increased collagenase and gelatinase activities in the medium of keratoconus corneal cultures. After the characterization of the matrix metalloproteinase (MMP) enzymes, studies focused on the expression of specific MMPs, in particular the gelatinases, MMP-2 and MMP-9. Matrix metalloproteinase-2 was found to be the major MMP of the cornea and was constitutively produced in normal tissue, whereas MMP-9 expression was induced by various stimuli, including phorbol esters and even tissue culturing. These studies suggested that there were no differences in the amounts or states of activation of MMP between normal and keratoconus corneas, although the amounts of some proteinase inhibitors, including tissue inhibitors of metalloproteinases, alpha-1-proteinase inhibitor and alpha-2-macroglobulin, were decreased in keratoconus. Most recently, the lysosomal proteinases, cathepsin B and cathepsin G were reported to be elevated in keratoconus corneas, and it is possible that it was cathepsin activity, not MMP activity, that was measured in some early studies. Nevertheless, there are now about 20 human MMPs identified and it is possible that some of these, other than the well known collagenase (MMP-1) and gelatinases (MMP-2 and MMP-9), could be implicated in the pathology of keratoconus. Studies have begun to address more recently described MMPs and it has been reported that the membrane-bound MT1-MMP (MMP-14), which activates latent MMP-2, was found to have increased expression in keratoconus corneas, whereas the stromelysins, MMP-3 and MMP-10, were not.