Numerical flow simulations have been performed on simplified artificial intracranial aneurysm models, as well as on real models obtained from rotational digital subtraction angiography images. The simplified geometries provide a basic understanding of the flow and allow the study of the influence of certain geometrical parameters with high accuracy and controllability. The real aneurysm models demonstrate that there is an infinite variety of shapes and very complex flow patterns. Nevertheless, it is proposed in this paper that the aneurysms can be divided into four basic classes corresponding to typical flow patterns. Some features of these have been identified and associated with possible causes of growth and rupture.