Because of the recent advances in embryo modeling techniques, and at the request of the Office of Science Policy in the Office of the Director at the National Institutes of Health, the National Academies of Sciences, Engineering, hosted a 1-day public workshop that would explore the state of the science of mammalian embryo model systems. The workshop, which took place on January 17, 2020, featured a combination of presentations, panels, and general discussions, during which panelists and participants offered a broad range of perspectives. Participants considered whether embryo model systems - especially those that use nonhuman primate cells - can be used to predict the function of systems made with human cells. Presentations provided an overview of the current state of the science of in vitro development of human trophoblast. This publication summarizes the presentation and discussion of the workshop.