There are numerous articles on the discovery of this arterial polygon, its history being quite long, beginning with the Antiquity and up to the Modern Era. Making an analysis of the primary and secondary sources on this topic, the purpose of this article is to identify the significant moments of the discovery of this arterial polygon, which is an anatomical structure with great importance for neurologists and neurosurgeons. We will present the contributions to this topic from Renaissance and early Modern Era anatomists, such as Andreas Vesalius, Jean Fernel, Gabriel Fallopius, Giulio Cesare Casseri, Adriaan van den Spiegel, Johann Vesling, and Johann Jakob Wepfer von Schaffhausen. We also pointed out that the contribution of the famous British anatomist Sir Thomas Willis (1621-1675) was the most important one. He published De Cerebri Anatome, in 1664, in which he described the vascular arrangement laying at the base of the brain, accompanying it by the exquisite drawings of Christopher Wren. Thus, he demonstrated to the medical world that he had reached the greatest understanding of the structure and function of the circular arterial anastomosis. For this excellent discovery, his followers honored him by giving his name to this arterial circle.