Dislocation, or luxation, of the lens arises as a result of rupture of the zonular fibers, which suspend the lens from the ciliary body. In the dog, lens luxation is most frequently encountered as a primary, heritable condition in which there appears to be an inherent weakness of the zonule. The condition is limited mainly to the Terrier breeds but has also been described in the Tibetan Terrier and Border Collie. The disease is essentially bilateral but seldom becomes apparent clinically before 3 or after 7 years of age. In most primary luxations, the lens passes into the anterior chamber and such cases must be regarded as emergencies on account of the likely development of secondary glaucoma; posterior luxations are usually less troublesome. Other causes of lens luxation include cataract formation, glaucoma, and uveitis. Congenital dislocations and those attributable to trauma alone are rare. In the cat, lens luxations are usually secondary and arise in later life.