The National Institutes of Health - and many other organizations and individuals worldwide - are interested in the state of the science on preventing Alzheimer's disease, mild cognitive impairment, and age-related cognitive decline. This letter report reviews the evidence on interventions to decrease the risk of developing clinical Alzheimer's-type dementia and mild cognitive impairment, and delay or slow age-related cognitive decline. It also makes recommendations that inform public health messaging on preventative interventions and recommendations for future research.