ATP-binding cassette transporters (ABC transporters) utilize the energy of ATP hydrolysis to translocate an unusually diverse set of substrates across cellular membranes. ABCA4, also known as ABCR, is a ∼250 kDa single-chain ABC transporter localized to the disk margins of vertebrate photoreceptor outer segments. It is composed of two symmetrically organized halves, each comprising six membrane-spanning helices, a large glycosylated exocytoplasmic domain located inside the disk, and a cytoplasmic domain with an ATP-binding cassette. Hundreds of mutations in ABCA4 are known to cause impaired vision and blindness such as in Stargardt disease as well as related disorders. Biochemical and animal model studies in combination with patient analyses suggest that the natural substrate of ABCA4 is retinylidene-phosphatidylethanolamine (N-retinylidene-PE), a precursor of potentially toxic diretinal compounds. ABCA4 prevents accumulation of N-retinylidene-PE inside the disks by transporting it to the cytoplasmic side of the disk membrane where it can dissociate, allowing the released all-trans-retinal to enter the visual cycle. The pathogenesis of diseases caused by mutations in ABCA4 is complex, comprising a loss-of-function component as well as photoreceptor stress caused by protein mislocalization and misfolding.