Posterior fossa (PF) malformations are commonly observed during prenatal screening. Their understanding requires knowledge of the main steps of PF development and knowledge of normal patterns in US and MR imaging. The vast majority of PF malformations can be strongly suspected by acquiring a midline sagittal slice and a transverse slice and by systematically scrutinizing the elements of the PF: cerebellar vermis, hemispheres, brainstem, fourth ventricle, PF fluid spaces and tentorium. Analysis of cerebellar echogenicity and biometry is also useful. This review explains how to approach the diagnosis of the main PF malformations by performing these two slices and answering six key questions about the elements of the PF. The main imaging characteristics of PF malformations are also reviewed.