Recent work has indicated that obesity increases the risk of developing all-cause dementia, Alzheimer’s disease and neurodegenerative processes in the brain. Given the current epidemic of obesity and the expected age-related increase in dementia incidence, even a modest association between these two conditions has far reaching public health implications. However, due to the effects of both Alzheimer’s disease-associated weight loss and age-related changes in body composition, there are methodological challenges in assessing obesity as a risk factor for developing dementia. There is a need to take a ‘life-course approach’ and to consider the role of risk factors prior to old age. In addition, there is value in assessing regional body shape distributions of adiposity. Several mechanistic pathways such as comorbid disease, leptin, adiponectin and inflammatory cytokines could explain the association between obesity and increased risk of dementia.