Angiogenesis is the formation of new blood vessels. Angiogenesis is a process controlled by certain chemicals produced in the body. Some of these chemicals stimulate cells to repair damaged blood vessels or form new ones. Other chemicals, called angiogenesis inhibitors, signal the process to stop. The study of natural and synthetic angiogenesis inhibitors, or antiangiogenic agents is important since tumors cannot grow or spread without the formation of new blood vessels.