Physiologic angiogenesis takes place during tissue growth and repair, during the female reproductive cycle, and during fetal development. Angiogenesis is also required for tumor growth and metastasis and, therefore, represents an exciting target for cancer treatment. Angiogenesis is a complex process that is tightly regulated by pro- and antiangiogenic growth factors. Pathologic angiogenesis is characterized by either excessive (eg. cancer) or inadequate (eg. coronary artery disease) neovascularization. Avascular tumors are severely restricted in their growth potential because of the lack of a blood supply. For tumors to develop in size and metastatic potential they must make an "angiogenic switch" through perturbing the local balance of proangiogenic and antiangiogenic factors. Frequently, tumors overexpress proangiogenic factors, such as vascular endothelial growth factor, allowing them to make this angiogenic switch. Two strategies used in the development of antiangiogenic agents involve the inhibition of proangiogenic factors (eg. anti-vascular endothelial growth factor monoclonal antibodies) as well as therapy with endogenous inhibitors of angiogenesis. Emerging antiangiogenic agents currently in clinical studies are discussed in this review.