In this paper, the major theories on the formation and growth of saccular aneurysms are reviewed on the basis of previous clinical, pathological and experimental studies. Which is the critical layer is still the focus of debate. Current ideas can be summarized as follows: most researchers think that disruption of the internal elastic lamina is an essential requirement for the creation of saccular aneurysms because it is this layer that provides most of the strength to the arterial wall, especially in cerebral arteries. Degeneration of this layer is a constant feature of all saccular aneurysms. A coexisting medial defect and haemodynamic stress at an apex may aggravate degeneration at that point. Hypertension does not seem to be the major factor in some aneurysm patients, but it may facilitate the formation and growth of saccular aneurysms. An atheroma is often associated with saccular aneurysms, but its effect on this pathological process is still unknown. Other factors are discussed concisely. Previous experimental methods and their results pertaining to the formation and growth of this type of aneurysm are also reviewed. A reproducible animal model is still required to allow various theories to be tested.