There are more than 36 million people in the US over the age of 65, and all of them are impacted by the cognitive decline and brain atrophy associated with normal aging and dementia-causing conditions like Alzheimer's disease, Lewy body disease, and vascular dementia. Recently, moderate exercise and improved fitness have been shown to enhance cognition in cognitively normal older persons as well as in individuals who complain of memory difficulty. Additionally, fitness correlates with brain volume in persons who are cognitively normal and those with Alzheimer's disease. Exercise in mouse models causes neurogenesis in the dentate gyrus. This review will discuss animal experiments, epidemiology, limited prospective studies, and biomarker data that make the case that prospective blinded studies are urgently needed to evaluate the role of aerobic exercise in protecting against dementia.