Arteries and veins are structurally different and have long been functionally defined by the direction of blood flow that they carry. However, a growing body of evidence indicates that the identity of the endothelial cells that line these vessels is determined in the developing embryo, before circulation begins. Recent work on the zebrafish has led to the identification of signals that are responsible for arterial and venous differentiation of endothelial cells, and highlights the unique benefits of this model organism in the study of vascular development.