The vascular endothelium is the interface in the cardiovascular system between the blood vessel wall and the flowing blood As such, these cells are exposed to both shear stress and circumferential stretch Though a lot is known about the regulation of gene expression by flow in mature vascular networks, very little is known in developing vessels Most vascular networks in the adult are homeostatic, exhibiting very low rates of endothelial cell replication and turnover In disease states such as cancer or macular degeneration, the vascular system is able to recapitulate embryonic growth and reinduce blood vessel growth The vasculature that develops is similar to the embryonic vasculature, and so many have used knowledge of embryonic development to interpret pathological blood vessel growth It was believed until recently that the embryonic vasculature was not sensitive to flow We recently showed that shear stress is necessary for proper vascular development We therefore review the role of blood flow and mechanical forces in vascular development We examine the pattern and magnitude of flow present in primitive vascular networks as well as exploring gene regulation by shear stress in both in vitro and in vivo embryonic systems