Every 3 seconds, someone in the world develops dementia. Mid-life obesity, diabetes, and hypertension are modifiable risk factors for dementia, whereas higher education level and physical activity may be protective. In this talk, I will discuss when and how these risk and resilience factors affect the ageing brain, using multi-modal MRI to characterise brain structure, function and vascular health. I will describe findings from the Whitehall II Imaging Sub-study and the EU Lifebrain Project (“Healthy Minds: 0-100 years”), a large collaboration of 13 European neuroimaging studies.