This chapter describes the vascular pathologies that underlie cerebral microbleeds (CMBs), concentrating on the two commonest disorders: hypertensive small vessel disease (SVD) and cerebral amyloid angiopathy (CAA). It also describes the process of blood degradation, and the correlation of imaging with histological findings. The chapter concentrates on hypertensive arteriopathy and CAA, which is usually diagnosed following symptomatic lobar intracerebral hemorrhage (ICH). Hypertensive arteriopathy is thought to be caused by a forced dilation of resistance vessels: that is, those vessels that regulate the blood supply volume to the distal capillary bed. The first event in a hemorrhage is the extravasation of all constituents of blood along with plasma. Extravasation may occur by rhexis (rupture of a vessel wall) or by diapedesis affecting arterioles, veins or capillaries. From the perspective of neuroimaging, CMBs are focal deposits of hemosiderin and can be visualized with MRI.