Small blood vessels are capable of continuously adapting their structural properties in response to the forces exerted on them by flowing blood and in response to the metabolic status of the surrounding tissue. Experimental studies of the structure and function of networks of microvessels have provided a basis for developing mathematical models for the dynamics of segment diameter and wall thickness in networks containing several hundred segments. From these models, the 'rules' governing the behavior of individual vessel segments can be deduced, such that the network meets the metabolic needs of the tissue adequately and efficiently. The results provide insight into the behavior of normal vascular networks and the perturbations occurring in disease states including hypertension and tumor growth.