Vascular endothelial growth factor (VEGF) is a homodimeric glycoprotein with a molecular weight of approximately 45 kDa. It is the key mediator of angiogenesis (the formation of new blood vessels), and binds two VEGF receptors (VEGF receptor-1 and VEGF receptor-2), which are expressed on vascular endothelial cells. In healthy humans, VEGF promotes angiogenesis in embryonic development and is important in wound healing in adults. VEGF is the key mediator of angiogenesis in cancer, in which it is up-regulated by oncogene expression, a variety of growth factors and also hypoxia. Angiogenesis is essential for cancer development and growth: before a tumor can grow beyond 1-2 mm, it requires blood vessels for nutrients and oxygen. The production of VEGF and other growth factors by the tumor results in the 'angiogenic switch', where new vasculature is formed in and around the tumor, allowing it to grow exponentially. Tumor vasculature formed under the influence of VEGF is structurally and functionally abnormal. Blood vessels are irregularly shaped, tortuous, have dead ends and are not organized into venules, arterioles and capillaries. They are also leaky and hemorrhagic, which leads to high interstitial pressure. These characteristics mean that tumor blood flow is suboptimal, resulting in hypoxia and further VEGF production. This central role of VEGF in the production of tumor vasculature makes it a rational target for anticancer therapy.