This chapter describes the development of knowledge concerning microaneurysmal and small hemorrhagic lesions in relation to intracerebral hemorrhage (ICH) and small vessel diseases, which may be relevant in understanding the origin of radiologically defined cerebral microbleeds (CMBs). The hypothesis that has arisen from the work of Jean-Martin Charcot and Charles-Joseph Bouchard is that pathological changes (degenerative or inflammatory, and often associated with arterial hypertension) in small caliber arterioles (up to approximately 300 µm) cause weakening of the wall and microaneurysm formation, which subsequently may rupture to cause ICH. Cerebral microbleeds are particularly common in ICH suggesting that they may provide clues to the underlying cause and could potentially shed light on the role of microaneurysms. Cerebral amyloid angiopathy is now recognized as a common disorder of elderly populations and is characterized by the deposition of amyloid in cerebral cortical and leptomeningeal small vessels.