Astigmatism is costly, prevalent, and unstable over time. Refractive error, which includes astigmatism, results in billions of dollars being spent annually on treatment in the United States. The overall adult prevalence of astigmatism >1 diopter (D) is ~45%, according to a multiethnic study of refractive error among 4430 adults. The definition of clinically significant astigmatism varies, but patients with corneal measurement of at least 0.5 D generally require visual correction. Features of astigmatism also shift with time; specifically, the anterior cornea usually exhibits with-the-rule (WTR) astigmatism in affected children and this changes to against-the-rule (ATR) astigmatism in elderly individuals. In contrast, the posterior cornea does not typically change significantly with age.