Recent work has indicated that obesity increases risk of all cause dementia, Alzheimer's disease (AD), and neurodegenerative processes in the brain. Given the current epidemic of obesity, and the expected age-related increase in dementia, even a modest association between these two conditions has far reaching public health implications. However, due to the effects of both AD associated weight loss and age-related changes in body composition there are methodological challenges in assessing obesity as a risk factor in the etiology of dementia. There is a need to take a 'life course approach' and to consider the role of risk factors prior to old age. Our work has indicated that obesity measured in middle-age is strongly associated with risk of dementia, independent of the development of diabetes and cardiovascular-related morbidities. This paper briefly reviews the evidence, and clinical implications of obesity as a risk factor for dementia.