Over the past 30 years, a number of studies employing the techniques of light and electron microscopy have been conducted on the corneas of human and animal eyes in an effort to more fully understand the histopathological basis of clinically-observed contact lens-related phenomena. This paper reviews studies that have sought to explain such effects, including epithelial abrasion, punctate staining and thinning; stromal oedema and thinning, and endothelial polymegethism. It is demonstrated that an understanding of contact lens-induced changes to corneal ultrastructure can have a significant impact on the clinical appreciation of the severity or otherwise of various conditions as observed with the slit-lamp biomicroscope.