The development of the vasculature is a complex process, the end result making it possible for the cardiovascular system to supply each tissue with the required amount of blood at the correct pressure. The mechanisms controlling this development are poorly understood, but in general the changes seen appear to be adaptive, so that the vessels perform their tasks more effectively. Increasing flow appears to be a powerful stimulus to growth, resulting in an increased lumen, whereas raised mean pressure and pulse pressure are associated with an increased media:lumen ratio. Thus, the physical environment (intravascular flow and pressure) plays an important role in determining vascular structure. This suggests that an important aspect of future research will be to understand more closely the mechanism by which the physical factors can stimulate growth, a process that must presumably be mediated by proto-oncogenes.