In recent years, modeling embryo development with stem cells has opened up new avenues in developmental biology. By reconstructing embryo-like structures using basic components — precise mixtures of embryo cell types, scaffolding with instructive signals, and induction with both transgenes and morphogens — these models allow us to untangle the self-organizing principles that coordinate embryo morphogenesis. An understanding that would otherwise be limited by the technical or ethical challenges associated with prolonged culture of human embryos.