Classification of ectatic corneal diseases is important not only for diagnosis, but also for treatment planning. There are three major entities that can be differentiated in terms of morphology and tomographic features; they are keratoconus (KC), pellucid marginal degeneration (PMD), and keratoglobus (KG). In addition, there are several terms under the name of KC, such as symptomatic, asymptomatic, progressive, non-progressive, keratoconus suspect (KCS), forme fruste keratoconus (FFKC), early keratoconus, established keratoconus, and pellucid-like keratoconus (PLK). Each of these terms has its definition and criteria. Moreover, grading of the ectatic diseases is essential for treatment planning, particularly in KC and PMD. The most clinically used grading systems are Amsler-Krumeich classification and its modifications, and Belin et al. categorization. This chapter overviews the new definitions and characterizations of the ectatic disorders, including classification and grading.