The embryology of the internal carotid artery (ICA) shows that this vessel comprises from origin to termination six segments, i.e. cervical, petrous, vertical cavernous, horizontal cavernous, clinoid and cisternal segments. Each of these segments displays a specific course and limits, defined by the origin of the following embryonic arteries: ventral pharyngeal hyoid, mandibular, primitive maxillary, trigeminal, dorsal ophthalmic and ventral ophthalmic. Each segment is independent and may show agenesis. In such cases the internal carotid blood flow (hemispheric arterial supply) is rerouted to afford usual ICA supply distal to the agenetic segment. All congenital anomalies of the ICA can be described and understood on the basis of embryological data. The "aberrant internal carotid" can therefore be identified as a normal vessel. Differentiation can be made between congenital versus acquired absence of the ICA. This type of analysis should allow the clinician to recognize what are normal, albeit rare variations, rather than to mistake them for an abnormal condition requiring treatment.