As more people live to old age, dementia is on the rise, but there is currently no effective treatment. This comes at enormous personal, societal and economic cost. I and others have previously shown that a decrease in heart function is associated with early cognitive impairment, however the underlying cause is not clear. It is possible that poor heart function may lead to poor blood flow in the brain, which may be the link. Another possible link between the heart and the brain runs in the opposite direction. Instead of heart problems leading to brain problems, it could be that lower cognitive function as a child leads to heart problems later in life. I will investigate the direction of the heart-brain association in 30yr olds from the ALSPAC cohort, that have had data collected since birth, including genetic, cognitive and heart measures. I will collect new detailed data on the heart and the brain during rest and during exercise. The results of this study are important as dementia is a public health priority. The clinical implications of reduced cognition among people with impaired heart function are considerable. By understanding the underlying mechanisms, we can reduce the burden of both of these conditions.