Endothelial cells are among the first differentiated cell types to arise during mammalian embryo-genesis. Thought to originate primarily from mesodermal progenitors, primordial (or primitive) endothelial cells are specified and organized into primitive vascular structures via the process known as vasculogenesis; intrinsic molecular signaling and microenvironmental cues such as hemodynamic stress drive angiogenic remodeling to subsequently shape the mature arteriovenous circulatory network. The developmental processes of vasculogenesis and angiogenesis are recapitulated postnatally during tissue growth, wound repair, and in the progression of prevalent cardiovascular diseases. Integrated knowledge of the molecular regulation of endothelial cell differentiation and blood vessel formation will thus not only further our understanding of normal vascular development, but also provide insights needed to establish and optimize clinical therapies. In this review, we discuss the origin and molecular regulation of endothelial cells from multipotent mesodermal progenitors, which may provide insights directly applicable to treating human disease.