Cerebral small-vessel alterations play a central role in determining lesions of subcortical structures and eventually may lead to cognitive impairment. Small-vessel diseases are classified according to the pathological viewpoint. The most important ones are the changes in small arteries and arterioles caused by prolonged hypertension. These small-vessel changes may result in ischemic damage to the brain parenchyma and blood-barrier alterations. Both mechanisms are thought to contribute to the occurrence of white-matter changes and lacunar infarcts. Modern magnetic resonance techniques such as diffusion, perfusion, and spectroscopy may allow the in vivo study of the pathophysiology of small-vessel diseases and the consequences on the brain parenchyma.