The basic morphology and neuroradiology of the prosencephalies are described. Differentiation of three groups is possible: 1. The dorsal sac category, 2. The intermediate category and 3. The pseudohemispheric category. Brains of the first group are entirely undivided into hemispheres, bowl-shaped and do have a dorsal sac. Those belonging to the intermediate category have lost the sac. Their posterior parts and the basal ganglia are increasingly cleft by incomplete interhemispheric fissure whereas the primitivity of the frontal region is fully preserved. Brains of the third category are pseudohemispheric i.e. there is a continuous, deep interhemispheric fissure the bottom of which is formed by an uninterrupted bridge for cingulate cortex instead of a corpus callosum. The fornices are lacking as well and the prosencephalic ventricles remain fused. The distribution of the cortical fields is similar to hemispheric brains but without ever achieving fully normality. The most important difference to lower category-brains is the complete separation of the motor cortex, and as regards gross morphology their superficial similarity to normal brains. The differential diagnosis from other monoventricular brains such as septum pellucidum defects and the callosal defects with interhemispheric cysts is discussed.