Since the first test-tube baby was born over 40 years ago, in vitro fertilization and other assisted reproductive technologies have advanced in extraordinary ways, producing millions of babies. An estimated 20 percent of American couples use infertility services to help them conceive, and that number is growing. Prospective parents routinely choose the sex of their future child, whether or not to have twins, or whether or not to pass on certain genes to the next generation, including those for chronic diseases, and probably soon, height and eye color. These rapidly developing technologies will require parents, doctors and policy makers to face critical questions about their use and possible misuse.