The increasing prevalence of obesity world-wide has an expected consequent increase in diabetes and cardiovascular disease. Less attention has been paid to the effect of obesity on dementia. This overview discusses methodological issues related to the epidemiologic study of obesity and dementia, reviews results of long-term prospective studies, and briefly considers possible mechanisms for an obesity-dementia association. At least six cohort studies of 18 to 32 years duration confirm that overweight middle-aged or older adults are at increased risk of dementia in later life. In many of these studies, the association persisted after adjusting for classical cardiovascular risk factors. A few epidemiologic studies (and more laboratory studies not reviewed here) suggest biomarkers such as C-reactive protein, interleukin 6, and leptin may explain part of the obesity-dementia connection. If any of these factors are in the causal pathway to dementia, their reversal or prevention by weight control would have huge public health importance.