The cell biology of the early processes of mammalian embryogenesis, such as germ-layer formation, has been technically challenging to study owing to the size and accessibility of mammalian embryos. Embryonic stem cells, which can generate the three germ layers in vitro, are useful for studying embryogenesis at the cellular level. So, how can the study of embryonic stem cells and their differentiation provide a deeper understanding of the cell biology of early development?