Brain imaging has played a major role in exploring abnormalities of brain structure and function in aging and dementia. Recently, with reports linking obesity to cognitive decline and dementia, magnetic resonance imaging has been used to investigate how brain structure may be altered with obesity. These studies have convincingly demonstrated both generalized and regional brain atrophy and changes in white matter in association with obesity. These results do not appear to be simply explained by links to cardiovascular disease. However, the mechanisms underlying these alterations are unclear and could be accounted for by a number of different processes that are known to alter brain structure and which could also be related to obesity. Application of additional imaging methods could help to establish the pathway through which obesity produces cognitive decline and dementia.