The epithelial to mesenchymal transition (EMT) converts epithelial cells into migratory and invasive cells and is a fundamental event in morphogenesis. Indeed, with the exception of ectodermal derivatives, notably the central nervous system and the epidermis, all vertebrate tissues are the result of one or several rounds of epithelial-mesenchymal transitions. In the embryo proper, the first EMT event occurs at gastrulation, when a subset of cells undergoes EMT to internalize and generate the mesoderm and the endoderm.Among the transcription factors that trigger bona fide EMTs, Snail genes are a good example to understand this process. Their role in development ranges from the subdivision of the early embryo into two main territories -ectodermal and mesendodermal- to the delamination of the neural crest and the formation of the heart valves. In addition to the induction of EMT, Snail can also control the cell proliferation and cell survival. I will discuss the roles of Snail genes during embryonic development and the effects of Snail deregulation both during the definition of embryonic territories and during organ development. As such, embryos with defective Snail at early stages cannot progress through gastrulation and an excess of Snail function during bone development leads to achondroplasia, the most common form of dwarfism in humans.