This workshop explored the state of the science of mammalian embryo model systems, including nonhuman primate and human models. The goal was to begin with an overview of the developments that have set the foundation for the field and then focus on opportunities and challenges for future work. Discussions included topics such as characteristics of advanced mammalian embryo model systems; differences between various mammalian embryo model systems and bona fide mammalian embryos; and differences between mammalian embryo model systems and mammalian “embryoid bodies.”