The emergence of CRISPR/Cas9 technology has revolutionized gene editing and made both gene therapy and eugenic control of future human evolution plausible. This accessible book puts these developments in their historical and scientific contexts and analyzes the policy and ethical challenges they raise. It presents the case for altering the human germ-line to eliminate a large number of genetic diseases controlled by a single or few genes, while pointing out that gene therapy is likely to ineffective for diseases with more complex causation. In parallel it explores the possibility of genetic enhancement in a similarly subscribed set of cases. But it also argues that, in general, genetic enhancement is ethically problematic and should be approached with caution. Given the success of CRISPR/Cas9 gene editing, and the explosion of related techniques, in practice it would be virtually impossible to ban germ-line editing for the future. A more useful goal is to regulate it with oversight that represents all stakeholders. That, in turn, requires an informed public discussion of these issues which this book aims to foster.