The central question of this book is whether or not particular cell entities of human origin ought to be considered human beings. The answer is crucial for making moral decisions for or against research and experimentation. Experts in the field discuss the production of embryonic-like pluripotent stem cells by altered nuclear transfer, parthenogenesis and reprogramming of adult somatic cells. They thoroughly analyse the biological and moral status of different cell entities, such as human stem cells, embryos and human-animal hybrid embryos, and make a decisive step towards establishing final criteria for what constitutes a human being. The topic is challenging in nature and of broad interest to all those concerned with current bioethical thought on embryonic human life and its implications for society.