The progressive growth of solid tumors is strictly dependent on their ability to attract new blood vessels that will supply them with oxygen and essential nutrients. Although in experimental models a number of compounds are able to elicit such angiogenesis, under normal physiological conditions new vessel growth is tightly repressed. One consequence of the cascade of oncogene activation and tumor suppressor gene loss that produces a solid tumor must be the alleviation of this normal repression. Understanding how this is achieved may help to define the functions of oncogenes and suppressor genes that are relevant to tumorigenicity and may suggest novel therapeutic strategies.