The cavum septum pellucidum (CSP) is an important fetal midline forebrain landmark, and its absence often signifies additional underlying malformations. Frequently detected by prenatal sonography, absence of the CSP requires further imaging with pre- or postnatal MRI to characterize the accompanying abnormalities. This article reviews the developmental anatomy of the CSP and the pivotal role of commissurization in normal development. An understanding of the patterns of commissural abnormalities associated with absence of the CSP can lead to improved characterization of the underlying spectrum of pathology.