Epithelium is the tissue phenotype of early embryos and primitive adults of the chordate phylum. A second tissue type, however, is produced by epithelial-mesenchymal transformation (EMT) in higher chordates, such as vertebrata. Mesenchymal cells have the ability, which true epithelia do not, to invade and migrate through the extracellular matrix (ECM) to create dramatic cell transpositions. The first-formed or primary mesenchymal cells in amniote vertebrates migrate from the primitive streak to differentiate into the mesodermal and endodermal epithelia. Definitive mesenchyme with connective tissue and muscle potentials arises from the epithelial mesoderm at about the same time as the neural crest mesenchyme forms from the ectoderm. Later on in embryogenesis. EMT is used to remodel unwanted epithelia, such as that of the palate medial edges. We discuss the mechanisms by which epithelial cells transform into mesenchyme and vice versa. On the one hand, cells activate putative mesenchymal master genes, turn off epithelial genes, and acquire motility machinery that allows them to interact in 3 dimensions (3D) with ECM via actin cortex while sliding their endoplasm into their new front ends. On the other hand, primary mesenchymal cells can reactivate epithelial regulatory genes, such as E-cadherin, turn off the motility machinery for invading ECM, and reexpress apical-basal polarity. We review the genes, such as FSP1, src, ras, and fos, that are activated in cells transforming to mesenchyme and the genes their neighbors activate to induce EMT, such as those for TGF beta, NT-3, and sonic hedgehog. Suspension in 3D collagen gels can induce adult epithelium to undergo EMT; alpha 5 beta 1 integrin is activated on surfaces in contact with collagen, including apical surfaces that do not normally express integrins. In vivo, it is possible that pathological manipulations of a cell's environment likewise induce EMT. Of the examples we give, the creation of invasive metastatic carcinoma cells by EMT is the most fearful. Interestingly, transfection of either metastatic cells or normal embryonic fibroblasts with the E-cadherin gene converts them to the epithelial phenotype. It may be possible in the future to manipulate the tissue phenotype of diseased cells to the advantage of the animal.