Hypoxia probably occurs to some extent in all solid tumors when oxygen consumption exceeds supply. It has long been recognized that cells become hypoxic as they are pushed away from blood vessels by the proliferation of oxygenated cells. Although necrosis eventually occurs, chronically hypoxic cells are of clinical importance as they are relatively insensitive to radiotherapy and chemotherapy yet capable of regrowth if reoxygenation occurs following the death of the oxic population. It is now recognized that hypoxia has wide-ranging effects, including the increased potential for invasion and metastasis that are not explained by this simple model. Consequently there is current interest in the effects of acute (or intermittent) hypoxia, due to temporal fluctuations in blood flow, and in adaptive mechanisms that lead to hypoxia tolerance and thus the accumulation of hypoxic cells. Tumor hypoxia is a dynamic process with major effects on cancer biology, of considerable current interest.