This chapter presents an overview of Merle ocular dysgenesis (MOD) that results from abnormal embryonic differentiation of tissues and may lead to multiple ocular abnormalities, including any combination of microphthalmos, microcornea, scleral staphyloma, persistent pupillary membranes (PPMs), heterochromia iris, iridal hypoplasia/coloboma, pseudopolycoria, iridocorneal angle dysplasia, lenticular coloboma/cataract/subluxation, choroidal hypoplasia, optic nerve coloboma, retinal dysplasia, and/or retinal detachment. Commonly affected breeds include the Australian Shepherd, (merled) Collies, Shetland Sheepdog, Harlequin Great Dane and (merled) Long-Haired Dachshund. The diagnosis of MOD is made on the basis of a combination of signalment and clinical findings. Surgical intervention in order to address scleral, lenticular, and/or retinal pathology may also be indicated in select cases. Selective breeding reduces the incidence and severity of disease.