Angiogenesis is the formation of new blood vessels from preexisting ones. Many solid tumors depend on an extensive newly formed vascular network to become nourished and to expand. Tumor cells induce the formation of an extensive but aberrant vascular network by the secretion of angiogenic factors. A proper context is needed for the endothelial cells to respond. To create this proper context, the tumor often uses the body's own repair system to accelerate angiogenesis and the subsequent tumor expansion. The angiogenic response is governed by the interaction of angiogenic growth factors and cytokines with specific receptors on the endothelium, as well as by the interaction of these cells with their surrounding matrix, which is regulated by matrix-degrading proteases and adhesion molecules such as integrins. A number of agents have been discovered and developed that aim to inhibit angiogenesis and to convert the tumor to a dormant state. They have proven to be effective in animal studies. At present their efficacy in man is under evaluation.