The vasculature is not simply a collection of hollow tubes through which blood flows - it is an organ in its own right, consisting of several cell types organized in a specific manner, and it mediates many unique biological and physiological functions such as selective filtration and angiogenesis. In tumors, blood vessels have structural and functional abnormalities that, to date, have generally hindered conventional therapies: the malformed networks make it difficult to deliver drugs uniformly to all cancer cells. Furthermore, flow in these vessels is chaotic, with intermittent stagnation followed by high-flow or even flow reversal in isolated segments. Cancer cells in stagnated or low-flow regions will receive suboptimal drug levels during chemotherapy. However, as more is learned about the formation of tumor vasculature and how it differs from that in normal tissue, more effective and directed therapies to fight cancer might be developed.