The ability of mouse embryonic stem (ES) cells to undergo differentiation in vitro complements their ability to contribute to numerous tissues in vivo and provides a unique model system for aspects of early mammalian development. ES cells are differentiated in two major ways: (i ) unmanipulated differentiation involves the removal of differentiation inhibitory factors, allowing the ES cells to undergo a programmed differentiation to form multiple cell types that provide developmental cues to each other; and (ii ) manipulated differentiation begins with the removal of differentiation inhibitory factors, but at some point the cells are usually disaggregated and cultured with specific added factors to purify or to increase the proportion of cells that acquire a particular developmental fate. Examples of both kinds of differentiation are found in this volume. The protocols provided here are for unmanipulated differentiation, which reproducibly results in the development of a primitive vasculature. Endothelial cells typically comprise 15–20% of the differentiated ES cells.