The human eye is an optical system with two lenses in parallel, with complementary functions essential to vision: the cornea and the lens. There is an intimate relationship between these two structures, especially during cataract surgery when there is inevitably corneal endothelium injury at different severity levels. Every cataract surgeon should be aware of the functioning of the fragile corneal tissue, especially its noblest layer and responsible for corneal transparency: the endothelium. It is of paramount importance to be able to identify the different endothelial pathologies and local conditions associated with greater tissue damage before cataract surgery, as well as to proceed individually in the pre-operative evaluation, during surgery (choice of supplies, technologies and techniques) and prescription of medications or need for additional procedures in the post-operative period. There are several conditions peculiar to cataract surgery and others to the corneal endothelium itself that are described and discussed, as well as information about the physiology, diagnosis and clinical and surgical treatment of diseases that affect it.