A review is given of the normal regulation of cerebral blood flow (CBF) and its pathophysiology in hypertension and stroke. In otherwise healthy hypertensive patients, the absolute level of CBF is the same as in normal subjects. CBF autoregulation, however, is shifted towards higher pressure, thus impairing the tolerance to hypotension. In most patients, this does not interfere with the beneficial effect of treatment, i.e., stroke prevention. Cerebral ischemia, however, may be provoked by overzealous pressure lowering in selected clinical settings: initial or intensified treatment of very severe hypertension, treatment of hypertension in the elderly, and treatment of hypertension in acute stroke. In the latter, a complicated sequence of brain ischemia and hyperemia makes antihypertensive intervention difficult in the early phase, when blood pressure is probably best allowed to decrease spontaneously.