The first requirement of a new prognostic indicator is that it should possess a clear biological significance. Indeed, much evidence shows that tumor growth and metastasis depend on neovascularization. Tumor angiogenesis (TA) refers to the growth of new vessels toward and within the tumor; unless tumor neovascularization occurs, cell proliferation reaches a steady state, and the tumor grows no larger than about 2 mm greatest diameter. Moreover, for tumor cells to metastasize, they must gain access to the vasculature from the primary tumor, survive the circulation, localize in the target organ, and induce angiogenesis in that target organ. TA is necessary both at the beginning and at the end of the metastatic cascade of events. Recently, my colleagues and I showed that a statistically significant correlation exists between incidence of metastases and microvessel density (MVD) of primary invasive breast carcinomas. Now, subsequent studies have shown that the association of prognosis with MVD exists not only in breast carcinoma but also in non-small-cell lung carcinoma, prostate carcinoma, and head-and-neck carcinoma. This article reviews the concepts and mechanisms of TA, the evidence supporting its role in growth and metastasis of solid tumors, and how measuring MVD within invasive tumors correlates with factor VIII-related antigen, blood vessel.