In eutherian mammals, the first cell types that are specified during embryogenesis are committed to form extraembryonic (placenta and fetal membranes) rather than embryonic structures. The trophoblast cell lineage, for example, forms at the morula-to-blastocyst transition: cells at the periphery of the morula become trophoblast, whereas cells on the inside remain undifferentiated embryonic ectoderm, which later gives rise to the fetus as well as the endodermal and mesodermal components of the placenta and extraembryonic membranes. Genetic studies in mice are beginning to identify growth factors and cell adhesion molecules that mediate interactions between cell types that are essential for morphogenesis of the placenta and fetal membranes, as well as transcription factors that control the differentiation of extraembryonic cell types.