National debates over embryonic-stem-cell research have divided citizens and lawmakers alike, as we seek a reasoned approach to these issues that honors our highest scientific and moral ideals. In this timely consideration of the nature and rights of human embryos, George and Tollefsen make a compelling case that we as a society should neither condone nor publicly fund research that kills or harms human beings at any stage of development including the embryonic stage. By looking rigorously at the science of embryological development and the moral norms that underwrite a just treatment of human beings, the authors make a purely scientific and philosophical case that the human embryo from the very beginning is a human being, entitled to all the moral and political rights inherent in that status. Just as none of us should be excluded from moral and legal protections based on race, sex, religion, or ethnicity, none should be excluded on the basis of age, size, or stage of biological development. George and Tollefsen forthrightly address the political, scientific, and cultural dimensions of the debate about embryo-destructive research and offer a summary of scientific alternatives to research that destroys or harms embryos. They conclude that the state has an ethical and moral obligation to protect embryonic human beings in just the same manner in which it protects human beings at later developmental stages. This revised, second edition includes an afterword addressing recent advances in stem-cell technology and appendix containing an exchange between the authors and William Saletan, who reviewed Embryo for the New York Times Sunday Book Review. Throughout, George and Tollefsen strive for the highest standards of analytical rigor and civil discourse in discussing this bitterly contested issue.