Recent advances in technology have improved stroke diagnosis, reduced the risks and increased the frequency of studies of stroke mechanism. Computer-assisted stroke data bank projects have provided new insights into the frequencies of stroke subtypes and the risks for progression and recurrence. A high frequency of strokes due to infarction remain unexplained despite thorough laboratory investigation. These infarcts of undetermined cause suffer recurrence rates almost as high as cardiogenic embolism, forcing a therapeutic decision even in the absence of a demonstrated cause. Stroke from atherosclerosis is far less common than formerly believed but carries the highest risk of worsening and early recurrence, prompting early treatment to attempt to avoid progression. Duplex and transcranial doppler methods of imaging blood vessels and insonating flow have now made it possible non-invasively to follow the course of atheromatous stenosis, embolism and recanalization, development of collateral flow, and vasospasm in ruptured aneurysms and arteriovenous malformations. Extracranial atheromatous disease may progress rapidly from mild to severe stenosis, stabilize at any point; intracranial collateral is not predicted by the degree of extracranial stenosis. Recanalization of cerebral embolism occurs early. Vasospasm after subarachnoid hemorrhage is common and often severe. Where available, magnetic resonance imaging is preferred to CT scanning for the diagnosis of every form of stroke including hemorrhage.