Tumors are characterized by an inefficient and disorganized vasculature which leads to tumor regions that are transiently or chronically undersupplied with oxygen (hypoxia) and nutrients (e.g., glucose). These adverse conditions are linked to treatment resistant and metastasizing disease with poor prognosis. Radiation sensitivity is dramatically lowered in hypoxic, yet viable and clonogenic, cells since oxygen is involved in the fixation of radiation-induced DNA damage (radiobiological hypoxia), and loco-regional tumor control is adversely affected in patients with hypoxic tumors. Hypoxia also leads to reduced sensitivity towards chemotherapeutics since drug delivery is reduced in hypoperfused hypoxic areas and hypoxic cells are quiescent, making drugs that target dividing cells ineffective. Fortunately, clinical attractive imaging and gene-expression based technologies that allows pre- and during treatment assessment of tumor hypoxia are now available. These technologies may identify patients suitable for established or emerging hypoxia-targeting treatments and, equally important; they allow us to monitor the efficacy of such intervention and may thus pave the way for effective individualized treatment. In the current review, we address 1) the causes and consequences of tumor hypoxia, 2) technologies that allow assessment of tumor hypoxia in individual patients and 3) current status of hypoxia-targeting treatments.