Quantitative determination of the degree of vascularity has been shown to be independently prognostically significant in many human tumor types. In particular, tumor vascularity has known importance in astrocytomas, in which endothelial proliferation is a criterion for anaplasia in many grading schemes. This chapter summarizes the known associations of quantitated microvessel parameters obtained from histologic sections of human brain tumors with clinical outcome, or other pathobiologic factors that have been examined. Among the conclusions are 1) brain tumors have the unique feature of complex "glomeruloid" vessels, as well as heterogeneity of microvascular distribution and caliber; 2) lower-grade astrocytomas incorporate pre-existing vessels, while glioblastomas develop new vessels; 3) quantitation may have additional independent prognostic value over and above routine histologic grade in low-grade astrocytomas with low tumor cell proliferative indices. These findings have implications for the appropriateness of antiangiogenic therapies