With the advent of efficient, easy-to-use genome editing by CRISPR-Cas9, editing human embryos is now possible, providing tremendous opportunities to study gene function and cell fate in early human development. The technique can also be used to modify the human germline. Unresolved questions about pre-implantation human development could be addressed by basic research using CRISPR-Cas9. In this Perspective, we discuss advances in human genome editing and consider ethical questions and potential clinical implications of this technology.