We present a novel approach to control endothelial tubulogenesis by spatially patterning cells within micromolded collagen gels. Endothelial cells cultured within microscale channels that were filled with collagen gel organized into tubes with lumens within 24-48 h of seeding. These tubes extended up to 1 cm in length, and exhibited cell-cell junction formation characteristic of early stage capillary vessels. Tube diameter could be controlled by varying collagen concentrations or channel width. The geometry of the microfabricated template also could be used to guide the development of branches during tube formation, allowing for the generation of more complex capillary architectures. Time-lapse imaging of tube formation revealed a highly dynamic process involving coalescence of endothelial cells, reorganization and alignment of collagen fibers into a central core, and arrangement of cells into cords. This platform may be of use to generate geometrically defined vascular networks for tissue engineering applications as well as a means to better understand the process of endothelial tubulogenesis.