The goal of this international, multicenter, mixed retrospective-prospective, exploratory, observational study is to establish an international registry to fill the knowledge gap created by the rarity of lenticulostriate and basilar perforator aneurysms and the resulting lack of knowledge about their natural history, diagnostic evaluation, management, and clinical outcomes. Specifically, comparative analyses of outcomes in patients with intracranial perforator aneurysms treated conservatively, endovascularly, and surgically will be performed. The study will also evaluate the size and location of lenticulostriate and basilar intracranial perforator aneurysms, collect information on the specific type of endovascular treatment used, compare patients with predominantly perimesencephalic subarachnoid hemorrhage (SAH) who have a perforator aneurysm to those without, and evaluate spontaneous occlusion in conservatively treated aneurysms. Core lab analysis of these aneurysms on various imaging modalities will be performed at the Inselspital and changes in aneurysm characteristics over time will be documented. In addition, the relationship between baseline characteristics and aneurysm characteristics on the incidence of hemorrhage will be examined. The study will involve approximately 20-30 centers worldwide with an estimated total sample size of 200-300 patients by 2025.