Often known as the window to the world, the cornea is the transparent anterior wall of the ocular coats that is most important in the visual pathway. Anatomically and physiologically, this layer is evolutionarily equipped to stay clear. Any diseased condition, be it congenital or acquired, can lead to loss of transparency and, therefore, visual loss. Once diseased, the cornea may never achieve its pristine original structure as healing occurs by scarring or opacification. Keeping with the objectives of this book, this chapter focuses on all the pathologies that affect the cornea highlighting the clinic-pathologic correlation with some case illustrations. The clinical signs and symptoms are the manifestations of the undergoing pathologic process, and a thorough understanding of each disease of the cornea is part of the learning objective of all clinical practice as well as clinical teaching programs of ophthalmology, optometry, and vision sciences.