No atlas of neurovascular anatomy would be complete without a description of the circle of Willis. The circle of Willis is known to almost everyone who has an interest in or who has studied the intracranial vascular system. The circle of Willis is a polygonal-shaped collection of arteries at the base of the brain that provides a pathway of communication between the anterior (carotid) and the posterior (vertebral-basilar) circulations. Images demonstrating the components of the circle of Willis are seen throughout this text. This collection of arteries is especially important in providing a pathway for collateral circulation in cases of intracranial vascular occlusions. Unfortunately a complete circle of Willis is present in only approximately 18% of the population. The “classical” circle of Willis includes both internal carotid arteries, both A1 anterior cerebral artery segments, both posterior communicating arteries, both P1 posterior cerebral artery segments, and the anterior communicating artery. In most cases one or multiple portions of the circle of Willis are developmentally/congenitally hypoplastic or absent. Common congenital variations include hypoplasia or absence of the A1 anterior cerebral artery segment (25%), hypoplasia or absence of a posterior communicating artery (32%), or hypoplasia or absence of the P1 posterior cerebral artery segment with fetal/direct origin from the ipsilateral supraclinoid segment of the internal carotid artery (15–22%). Additionally, the anterior communicating artery may be fenestrated or absent (9%). An incomplete circle of Willis is the norm rather than the exception and often results in the isolation of a vascular territory.