Epithelial-mesenchymal (E-M) interactions are a series of programmed, sequential and reciprocal communications between the epithelium and mesenchyme (two heterotypic cell populations) resulting in the differentiation of one or both the cell populations involved. In the developing embryo, a functional relationship exists between epithelium and the mesenchyme that supports it, and the proper interplay is essential for orderly development. Development of most vertebrate organisms which are composed of an epithelium and mesenchyme requires extensive precisely regulated communication between these tissues.Epithelial-mesenchymal interactions are necessary for the development of oral epithelium during embryogenesis, but adult oral epithelium is also under the influence of mesenchymal tissue in both normal and neoplastic conditions. Reciprocal E-M signaling has been shown to be involved at different stages of tooth development. Development of tooth has been intensely studied as a model system for E-M interactions during organogenesis.