The normal thickness and transparency of the cornea is maintained by the barrier function and the active fluid pump of the corneal endothelium. The major barrier is the intercellular gap junctions, and the fluid pump depends on the active transport of bicarbonate ions. Three methods are currently available for studying this important cell layer. (1) Endothelial dysfunction produces corneal swelling, and measuring the thickness of the swelling permits the degree of damage and the repair processes to be evaluated. (2) The endothelium's permeability to fluorescein reflects the state of its barrier. (3) The morphometric measurements obtained by specular microscopy of the endothelial cells permit the cell size distribution pattern and any alterations in it to be studied. Although the cell size distribution is normal in the young adult, cellular pleomorphism increases and cell density decreases during the aging process. A follow-up study of cell transformation after surgical trauma disclosed that the human endothelium shows practically no proliferative activity and that the damaged area is covered by means of cell migration. This migration, however, is incomplete, resulting in persistent regional differences in the cell distribution pattern. The traumatized endothelium continues to lose cells at an accelerated rate and endothelial dysfunction may develop many years after injury.