This study was carried out to describe the main variations of the Circle of Willis and to clarify the clinical importance of these variations, so that practitioners and the students are more alert when dealing with this subject.This work is based on the study of 50 Circles of Willis from adult Jordanians who died from diverse causes without signs of cerebrovascular diseases. The arteries of these circles are drawn and measured with the aid of a stereoscopic microscope provided with a drawing tube.Fourteen types of Willis polygonals have been identified among which 13 would be assimilated to polygonals already described in previous studies. The 14th is original and seems proper to our series. We have described wide diversity of variations found within these 14 types of Willis polygonals. We presented the frequency of the different variations and discussed the clinical significance of these variations.The variations of the Circle of Willis are very common and they may have important influence on the manifestations and treatment of some diseases that are caused by obstruction of one of the arteries that supply the brain.