Brain malformations have been classified on the basis of the types of errors which occur during embryologic development. The basis of the classification is the level of formative pathogenesis at which the error occurs. Development can be artificially regarded as consisting of cytogenesis, histogenesis, and organogenesis. Although the final stage of a malformation can be classified under one of these headings, the categories are not mutually exclusive. The classification is given in the form of a dendrogram which permits any further degree of specificity or subdivision desired.