Copper is a trace element which is tightly regulated in mammals and lower animals. Disruptions of copper homeostasis in humans are rare and they cause serious disorders such as Wilsons disease and Menkes disease. Copper plays an important role in promoting physiological and malignant angiogenesis. Formation of new blood vessels by a tumor enables tumor growth, invasion, and metastasis are copper requiring processes. The copper chelator tetrathiomolybdate (TM), which quickly and effectively depletes copper stores, is under investigation as an anti-angiogenic agent. Promising results from in vitro experiments, in pre-clinical animal models, and in a phase I clinical trial have led to several phase II trials of TM in patients with advanced cancers.