Neural tube defects represent sequela of incomplete fusion events, deficient neural proliferation or a failure of dorsal mesenchymal induction events involved in the formation and maturation of the embryonic neural tube. Much of the pathology is consequent on the extent, timing and location of such developmental errors, as well as any accompanying secondary events. Failure of neural tube closure results in open neural tube defects, including craniorachischisis, anencephaly, some encephaloceles, rachischisis, myeloceles and myeloschisis. Closure with deficient proliferation or dorsal induction results in closed neural tube defects, including some encephaloceles, meningoceles, and myelocystoceles. Secondary events include inflammation due to exposure to vernix, infarction, haemorrhage or hypoplasia of herniated tissue, and syrinx development. Chiari malformations are discussed in Chapter 7.