Individuals, families, and societies around the world are concerned about dementia and the other forms of cognitive impairment that affect many older adults. It is now known that brain changes typically begin years—if not decades—before people show symptoms, which suggests that a window of opportunity exists to prevent, slow, or delay the onset of these conditions. Further, emerging evidence that the incidence and prevalence of dementia are declining in some high-income countries offers hope that public health interventions can be effective in preventing cognitive decline and dementia. Although the evidence base on how to prevent, slow, or delay these conditions has been limited at best—despite the many claims of success made in popular media and advertising—a growing body of prevention research is emerging. A systematic review published in 2010 by the Agency for Healthcare Research and Quality (AHRQ) and an associated “state of the science” conference at the National Institutes of Health (NIH) concluded that there was insufficient evidence to make recommendations about interventions to prevent cognitive decline and dementia. Since then, understanding of the pathological processes that result in dementia and cognitive impairment has significantly advanced, and a number of clinical trials of potential preventive interventions have been completed and published, with more under way or being planned. Within that context, the National Institute on Aging (NIA) initiated this study with the National Academies of Sciences, Engineering, and Medicine to take stock of the current state of knowledge on interventions for preventing cognitive decline and dementia, to help shape the messages NIA conveys to the broader public about these conditions, and to inform future actions and research in this area. The expert committee was tasked to help inform the design of a new AHRQ systemic review whose results then would be used by the committee as the primary evidence base for recommendations on the appropriate content for communicating with the public about steps that can be taken to prevent, slow, or delay the onset of mild cognitive impairment (MCI) and clinical Alzheimer’s-type dementia (CATD) and delay or slow age-related cognitive decline (ARCD), as well as recommendations for future prevention research.