This paper presents an updated analytic review of the holoprosencephalic disorders. Central nervous system findings and craniofacial anatomy are discussed in depth. Associated anomalies and syndromes are updated with a syndrome commentary that attempts to separate durable syndromes from associated anomalies and spurious entities. The differential diagnosis of various conditions that may simulate the holoprosencephalic disorders is also discussed. Various experimental approaches to holoprosencephaly are reviewed, indicating that holoprosencephaly appears to be both etiologically and pathogenetically heterogeneous.