Astigmatism is a common refractive error caused by the difference in refractive power of the eye along different meridians. This causes two line foci that cannot be corrected by changing viewing distance or accommodation. Although human studies have ascribed astigmatism to multiple factors, its cause remains unclear. Studies in chicks and monkeys suggest that imposed astigmatic error may alter emmetropization, but McLean and Wallman (2003) showed that the early compensatory response to spherical defocus was not affected by concurrent high astigmatism in chicks. This review will focus on possible mechanisms leading to astigmatism and the influence of astigmatism on emmetropization in animal studies.