Outermost layer of the eye is cornea which is very clear and transparent. Many diseases can affect the cornea in that Terrien's marginal degeneration is also one of them, and their causes are unknown. But it is associated with other diseases like scleritis, episcleritis, and keratoconus, and this condition can be diagnosed by corneal topography, slit-lamp microscopy, ultrasound biomicroscopy, and scanning-slit topography. Medically, it can be treated by contact lenses and surgically by different types of keratoplasties.