Recent advances in mammalian gamete and embryo micromanipulation have stimulated the scientific and medical communities, and to some degree the public at large, to become aware that treatment of genetic disease by direct alteration of the genetic code may soon be possible. Because these micromanipulation techniques result in modification of the genotype at the earliest stages of development, such "gene therapy" affects not only the conceptus itself but also its germ cells. Thus such genetic modifications are heritable and can be transmitted indefinitely to succeeding generations of progeny. In the presentation, both narrow and broad definitions of gene therapy will be considered with respect to the techniques upon which they are based, their potential for treatment of genetic disease, and their current feasibility.