Collateral blood vessels develop by growth of pre or newly formed structures in almost all vascular provinces as a consequence of progressing stenosis of the main artery. These alternative routes of blood supply are potentially able to alter the course of vascular disease. Collateral development is a time consuming process, and arterial stenosis and occlusion often progress faster than growth of the alternative routes. The authors' ultimate goal is to provide a better understanding of collateral growth in order to pave the way for improving the conditions for these potentially selfhealing processes. These were programmed by nature but have not been perfected, probably because defenses against arterial disease had not been put under the pressure of natural selection.