In this chapter, we will review the formation of the definitive endoderm, the population of cells that give rise to the lining of the digestive tract, its associated organs and the pharyngeal pouches. At the cellular level, we will describe the location and movement of endodermal cells from the onset of epiboly until the end of gastrulation. At the molecular level, we will discuss the genes associated with endoderm formation beginning with Nodal signaling. For convenience, we use the term involution, sometimes referred to as internalization; we also separate endoderm formation into the pre-involution (blastula) and post-involution (gastrula) periods although of course endoderm formation involves a continuous series of events. In addition, we refer to the cells that contribute to the endoderm as progenitors prior to their involution and precursors after their involution.