The normal level of the corneal thickness and hydration is maintained by the barrier function and active fluid pump of the corneal endothelium. Three methods are currently available for the clinical study of this cell layer: (1) endothelial dysfunction results in a corneal swelling, and measurement of the corneal thickness allows the degree of endothelial damage and its repair processes to be evaluated; (2) the state of the barrier function may be studied through determination of the endothelial permeability to fluorescein; and (3) the endothelium can be photographed by specular microscopy, and the cell density and cell size distribution can be studied by morphometric analyses. A follow-up study of the cell transformation after surgical trauma revealed that the human endothelium shows very little proliferative activity, and the damaged area is covered by migration of cells in the surrounding area. It appears that the traumatized endothelium loses cells at a faster rate than that seen in the normal aging process, and endothelial dysfunction may develop many years after injury.