In continuation of earlier studies concerning the aneurysms of ICA and PCA a proposed detailed classification of vertebro-basilar aneurysms has been given. It has been proved that nearly all the aneurysms originate at the divisions of the main arteries or at the ends of large arteries as direct continuations of the main branches (BA as well as ICA). Aneurysms of the vertebro-basilar junction are of special aetiological interest. These last aneurysms are unique, and demonstrate that congenital defects seem to be more important for the origin of aneurysms than the direction of the blood flow. Origin and projection characterize the different types. Atypically located aneurysms away from arterial divisions exist, as in the ICA system. The relations between aneurysms and the multiplicity of variations and anomalies of the main arteries and the posterior part of the circle of Willis are still unknown, and should be checked in detail. Our studies have convinced us that the exact knowledge of microtopographic classification is the key to correct preoperative diagnosis, access, and treatment.