Pellucid marginal degeneration (PMD) is a rare type of corneal ectasia of unclear etiology. Typically characterized by inferior corneal thinning with a superior protrusion that spares the central cornea, it is the second most common corneal ectasia after keratoconus [1, 2]. Epidemiologically, PMD tends to affect men more than women [3], can occur in the second to fifth decade of life [1, 3], and is commonly bilateral [1, 3]. Vision decreases proportionally with worsening astigmatism. Pathology studies on PMD demonstrate a disruption of Bowman’s layer associated with stromal thinning, with otherwise normal corneal structures [4]. On a molecular level, collagen abnormalities may also play a role in the pathophysiology [5]. The overall diagnosis of PMD is primarily clinical based on the slit lamp exam with supporting topographic or tomographic evidence (Fig. 15.1). Once advanced, PMD can lead to acute hydrops or spontaneous corneal perforation. Corneal scarring often follows hydrops in patients with PMD.