Hypertension is the most important risk factor for stroke and increases the risk of dementia. The brain itself also plays an important role in regulating blood pressure, which appears to be impaired in patients with cerebrovascular disease and dementia, potentially leading to a vicious cycle accelerating progression of the cerebral pathology. However, although the mechanisms by which increased blood pressure causes stroke and vascular dementia are poorly understood, the concept of dysregulation of BP has been relatively neglected and another concept has come to dominate research and practice – that each of us has an underlying “usual” BP, which is the main determinant of BP-related vascular risk and of benefit from BP lowering drugs. BP is often highly variable in patients at risk of stroke and vascular dementia, but guidelines recommend that diagnosis and treatment of hypertension should be based on estimates of the “true” underlying mean BP. However, there is evidence that patients with only episodic hypertension also have a high risk of stroke, that maximum BP may be a better predictor of stroke than mean BP, that residual visit-to-visit variability in BP on treatment has a poor prognosis despite good control of mean BP, and that benefits of some BP-lowering drugs are due partly to reduced variability in BP and better control of maximum BP. Compared with other drugs, variability in SBP and maximum level is reduced by calcium channel blockers (CCBs) and non-loop diuretics than by ACE-inhibitors angiotensin-receptor blockers and beta-blockers. These findings could explain why beta-blockers and ACE-inhibitors are less effective, and CCBs and diuretics are more effective, in preventing stroke than can be accounted for by effects on mean BP. Increased long-term mean BP is undoubtedly an important risk factor for stroke and vascular dementia, but there is increasing evidence that maximum BP (and hence variability and instability in BP) also plays a role in the progression of organ damage and in triggering cerebrovascular events.