What exactly is an embryo? In German law, the embryo is defined according to the criterion of developmental biological totipotency. But when somatic cells are reprogrammed into induced pluripotent stem cells, it can at present not be completely ruled out that these cells undergo a temporary totipotent stage. The contributions to this volume examine what consequences arise out of this hypothetical assumption for the biological significance and the moral and legal assessment of human embryos.Setting out from the presentation of the historical concept and current interpretations of the term developmental biological totipotency, the legal section of the volume examines to what extent totipotency is an adequate criterium for the legal definition of the embryo, or whether it needs to be amended. Finally, the philosophical section reflects on the definition of the embryo in terms of potentiality, naturalness and purpose.