Diffusion-weighted MRI (DWI) has already had a substantial effect on the diagnosis of patients with ischaemic stroke. It provides in vivo pathological information and allows the differentiation of acute stroke from chronic stroke and from non-specific white-matter lesions. The high contrast of the acute DWI lesion against the dark background facilitates the detection of lesionseven when they are 1 mm or less in diameter. Small lesions, which are undetectable by other means, include small lacunar infarcts, punctate cortical infarcts, and DWI bright dots in patients with transient ischaemic attacks (TIA). The latter constitute remnants or “footprints” of recent ischaemia and confirm the clinical TIA syndrome as ischaemic. Because of these attributes, DWI not only confirms the clinical diagnosis, but also facilitates the recognition of certain patterns of ischaemia, thereby providing clues to the underlying aetiology. DWI is becoming an important technique for optimum management of patients.