The study of human development is essential to further our knowledge and to improve our therapeutic strategies in the fields of reproductive and regenerative medicine, but access to human embryos is limited and the creation of fresh ones for research is prohibited. Alternative strategies thus need to be developed to overcome these limitations. One of these strategies is the chimera technology using pluripotent stem cells, including human/animal chimeras. I will explain the importance of this experimental paradigm for studying human development. I will then discuss the biological issues raised by interspecies chimeras, our recent results, and the ethical issues raised by these new technologies.