The molecular regulation of smooth muscle differentiation is currently far less well understood than that of striated muscle, in part because in this cell type, the differentiated state is plastic and reversible. In recent years, however, several molecules, the best characterized of which is myocardin, have been shown to be necessary and sufficient to promote at least partial smooth muscle differentiation. Indeed, mice deficient in myocardin have a severe reduction of smooth muscle tissue. However, possibly because of multiple embryological origins, which include mesenchyme, neural crest, and even endothelium, different types of smooth muscle cells differ in their expression of myocardin and of other potential regulatory molecules. Here, we will review recent work on the topic, focusing on the mesoangioblast, a recently described vessel-associated stem cell, whose differentiation into smooth muscle is dependent upon expression of msx2 and necdin, but not of myocardin.