 Despite often robust and durable clinical responses, essentially all patients treated with bevacizumab progress, implying primary or acquired resistance, or both. Treatment related changes in VEGF ligands and receptors have been reported in response to anti-VEGF therapy in preclinical models. Treatment related changes have also been noted for many additional angiogenic factors, including PDGF, bFGF, and IGF, all of which have been shown to potentially mediate sensitivity and resistance to anti-VEGF therapy. Despite the known importance of numerous angiogenic factors in both preclinical models and in patients, there have not yet been a broad and systematic evaluation of either baseline or treatment related changes of multiple angiogenic factors in response to bevacizumab. Similar information is also lacking for essentially all targeted therapies. 