The fact that a single blood vessel can support the life of thousands of tumor cells has been known for a long time. However, therapeutic strategies that aim to impair vascular development in tumors are only slowly emerging in the clinics. Nevertheless, the accumulation of data from many successful preclinical studies of the effects of a variety of drugs that target tumor vasculature provides clues that should help rationalize future treatment modalities for human tumors. Indeed, the 'old' view of an immature and non-functional vascular network within tumors has evolved and, in this article, we will show that the concept of tumor heterogeneity should be extended to the vascular compartment. In addition, we will review recent data documenting that both mature and immature vessels coexist within tumors and, importantly, that their relative density responds to a dynamic process that evolves with time and treatments.