The period established embryonic stem cells as a tractable, pluripotent resource, enabling long-term culture and sustained self-renewal in vitro and providing a platform for multilineage differentiation studies. Foundational demonstrations showed pluripotent lines could be derived from early mouse embryos and maintained in culture, creating a new era of developmental genetics and in vitro modeling. Leukemia inhibitory factor emerged as a key tool for preserving developmental potential, while promoter traps introduced effective genetic screens in embryonic stem cells. Historical Significance: Collectively, these breakthroughs defined a paradigm in which mammalian pluripotent cells could be captured, cultured, and genetically analyzed, reshaping understanding of mammalian development. The formation of embryoid bodies that recapitulate endoderm, mesoderm, and ectoderm in vitro provided a powerful model of early development. These advances laid the groundwork for future stem cell research, regenerative medicine, and functional genomics in mammals, beyond the initial mouse studies.