Leukoaraiosis describes diffuse white matter abnormalities on CT or MR brain scans, often seen in the normal elderly and in association with vascular risk factors such as hypertension, or in the context of cognitive impairment. The term was introduced to avoid confusing an imaging appearance with a specific pathology. Neurologists often come across this appearance, but its significance is sometimes uncertain. The effects on cognitive function and gait are insidious and can be difficult to detect at the bedside, but are nevertheless important. However, gradually it is becoming clearer how leukoaraiosis relates to cerebrovascular disease, Alzheimer's and other diseases, and how this appearance should influence treatment decisions.