Diabetes mellitus has a significant effect on morphological, metabolic, physiological, and clinical aspects of the cornea. Morphological changes are manifest in the corneal epithelium, epithelial basement membrane and basement membrane complexes, stroma, and endothelium. The homeostasis of these structures can be altered by diabetes in both the non-stressed and the stressed cornea, causing myriad primary and postoperative manifestations. The polyol pathway appears to be involved in some of the pathophysiology mechanisms leading to these clinical entities and appears also to play a role in the treatment of some diabetic pathological processes associated with diabetes mellitus.