Cerebral aneurysms are an enlargement of a vessel or part of a vessel, still lined by endothelium. Primarily arterial, they may be roughly spherical in shape ("berry" aneurysms), or tubular/fusiform (dolichoectasia).

Aneurysm classifications:
* congenital (e.g. berry aneurysm)
* mycotic (infection or neoplastic wall damage)
* hypertensive (e.g. dolichoectasia)
* traumatic (e.g. Ant. cerebral artery)

The most common type is the "berry" aneurysm, that is usually seen in the larger vessels of the skull base (circle of Willis) arising from the natural weakness at the vessel bifurcation (branch point).

Aneurysms larger than 25 mm are called "giant".

Aneurysms may be complicated by rupture (causing subarachnoid hemorrhage - SAH); rupture into the brain causes a parenchymal hematoma; SAH can cause vasospasm that may lead to infarction; may be large enough to have internal thrombus (that can embolize); SAH may lead to hydrocephalus (by reducing CSF reabsorption); and, may exert mass effect on surrounding structures.

In the past, most aneurysms were usually treated by "clipping" - a neurosurgical procedure to place a clip across the "neck" of the aneurysm that then isolates it from the parent vessel while preserving distal flow.

Complications of aneurysm clipping include vasospasm and ischemia/infarction. Removing subarachnoid clot has been shown to improve clinical outcomes.

Interventional radiology, using endovascular therapy with thrombogenic coils - has gained acceptance as a less invasive therapy compared to craniotomy and clipping.

The most common venous aneurysm is an enlargement of the vein of Galen (VOG) - a midline vessel in the quadrigeminal (plate) cistern. VOG enlargement is usually due to a shunt (AVM or AVF) that increases the flow and pressure in the deep venous system. When this occurs during fetal development, there is often persistence of the prosencephalic vein of Markowsky.