The objective of this article was to summarize current knowledge of blood flow and oxygen supply to human tumors, parameters which go hand in hand, and in turn critically determine the cellular metabolic microenvironment of human malignancies. A compilation of available data on blood flow, oxygen supply, and tissue oxygen distribution in human tumors is presented. Though data on human tumors in situ are scarce and there may be significant errors associated with the techniques used for measurements, experimental evidence is provided for the existence of a compromised and anisotropic blood supply to many tumors. Comparable to rodent tumors, O2-depleted areas develop in human malignancies which coincide with nutrient and energy deprivation, and with a hostile metabolic microenvironment. Significant variations in these relevant parameters have to be expected between different locations within the same tumor, at the same location at different times, and between individual tumors of the same grading and staging.