This research summarises a research program that is concerned with the effects of high fat diets on cognitive function in rats. The diets selected accurately represent current upper limits of human fat consumption in western societies. Rats fed with diets high in saturated or unsaturated fat for 3 months, were severely impaired on a range of learning and memory tasks. Related studies showed that these effects were modulated by concentration of fat, environmental influences, and treatment with glucose. More work is needed to identify the neurobiological mechanisms underlying this impairment but saturated fatty acid intake, as well as insulin resistance and glucose intolerance may be important factors. In demonstrating a clear relationship between obesity and cognitive impairment, this research has important implications for aging. Older adults are particularly vulnerable to the adverse effects of dietary fat and the present results underscore the importance of evidence that seniors with marginal levels of nutrient intake often perform poorly on tests of cognitive function, and are at increased risk for various forms of dementia.