The newly released Institute of Medicine report, Review of NASA’s Human Research Program Evidence Books: A Letter Report, examines NASA’s plans to assemble the available evidence on human health risks of spaceflight and to move forward in identifying and addressing gaps in research. The committee provided recommendations to strengthen the content, composition, and dissemination of the evidence books. Opportunities to further improve future versions of the evidence books are discussed so that these documents can serve as a continuously updated knowledge base of evidence regarding risks to human health associated with spaceflight, particularly spaceflight beyond low Earth orbit and of long duration.