The mesenchyme is a remarkably plastic tissue in the embryo. Recent studies have led to the discovery of mesenchymal cells in the adult organism that can differentiate in vitro into unexpected directions, beyond the well-known ability of the mesenchyme to give rise to mesodermal derivatives. These studies highlighted the plastic nature of the mesenchyme, also beyond the embryonic developmental stage. This review discusses the possible functions of the mesenchyme in the adult and the reason for the maintenance of plasticity throughout mammalian life. The properties of the mesenchymal cells clearly exemplify the stem state concept; cells, whether early or late in the differentiation cascade, may assume a stem state that entails high plasticity.