Aplasia cutis congenita is a condition in which localized or widespread areas of skin are absent at birth. Several distinct clinical subtypes occur, characterized by the location and pattern of skin absence, the presence of associated malformations, and the mode of inheritance. The disorder is seen most frequently on the scalp, often as a solitary lesion without other anomalies. Scalp lesions can be seen in association with limb reduction defects and in association with epidermal and organoid nevi. Lesions may overlie overt or occult embryologic malformations. A form of aplasia cutis congenita occurs in association with placental infarcts or the in utero death of a twin fetus. The condition may be associated with epidermolysis bullosa, specific teratogens or intrauterine infections, or it may occur in the presence of chromosomal abnormalities, ectodermal dysplasias, or other syndromes of malformation. A classification for aplasia cutis congenita is proposed.