New biochemical tools have made it possible to change the DNA sequences of living organisms with unprecedented ease and precision. These new tools have generated great excitement in the scientific and medical communities because of their potential to advance biological understanding, alter the genomes of microbes, plants, and animals, and treat human diseases. They also have raised profound questions about how people may choose to alter not only their own DNA but the genomes of future generations. To explore the many questions surrounding the use of gene editing tools in humans, the U.S. National Academy of Sciences, the U.S. National Academy of Medicine, the Royal Society, and the Chinese Academy of Sciences hosted an international summit in December 2015 to present and deliberate on the scientific, ethical, legal, social, and governance issues associated with human gene editing.