We are in the midst of a revolution in medicine: human genetic engineering. Like earlier revolutions in health care, such as surgery with anesthesia or the use of antibiotics, genetic engineering has the potential to greatly advance the health and wellbeing of mankind. Yet unlike earlier innovations, human genetic engineering raises serious ethical questions. It may be one thing for an adult to undergo gene therapy to cure a disease, but what about modifying human embryos to prevent that disease? And if embryos can be altered to improve health, what about to improve intelligence or to select physical characteristics such as hair or eye color?