With an extraordinary new technology called CRISPR, we can now edit DNA—including human DNA. But how far should we go? Gene editing promises to eliminate certain genetic disorders like sickle cell disease. But the applications quickly raise ethical questions. Is it wrong to engineer soldiers to feel no pain, or to resurrect an extinct species? And is there harm in allowing parents to choose their child’s features, like eye color or height? The scientists who pioneered human genome studies and CRISPR grapple with these questions.