Congenital absence of the internal carotid artery (ICA) is an extremely rare entity that occurs due to insult during the embryonic development of the ICA. Various intracranial collateral pathways develop to compensate for the ICA agenesis. Patients can present with aneurysmal subarachnoid hemorrhage, stroke-like symptoms, or other neurological symptoms due to compression of brain structures from enlarged collateral pathways/aneurysms. We present two cases of ICA agenesis along with an extensive review of the literature. A 67-year-old man presented with fluctuating right-sided hemiparesis and aphasia, found to have left ICA agenesis. The left middle cerebral artery (MCA) is supplied by the basilar artery through the well-developed posterior communicating artery (PCOM). Left ophthalmic artery coming from the proximal left MCA. A 44-year-old woman presented with severe headaches, found to have right ICA agenesis with bilateral MCAs and anterior cerebral arteries (ACA) supplied by left ICA. A 17-mm anterior communicating artery (ACOM) aneurysm was discovered.