Mammalian systems activate a number of adaptive responses during hypoxia that protect the organism from the consequences of severe oxygen deprivation. These responses are evident at the organismal level, the tissue level, and at the cellular level. They include transcriptional activation, and Hypoxia Inducible Factors-1 and -2 (HIF-1 and -2) are critical for the activation of genes encoding glycolytic enzymes, membrane glucose transporters, vascular growth factors, erythropoietin, and nitric oxide synthase. Post-translational responses to hypoxia include neurotransmitter release, alterations in membrane ion transport, and changes in vascular smooth muscle tone. In the lung, hypoxic pulmonary vasoconstriction (HPV) helps to optimize gas exchange but it also contributes to the development of pulmonary hypertension in hypoxic lung diseases. Much is known about the signaling pathways mediating the molecular responses to hypoxia, but the mechanisms of oxygen sensing underlying transcriptional and post-translational responses are not well understood. Putative models that may act as cellular oxygen sensors mediating HPV include the NAD(P)H oxidase family, which should decrease the production of reactive oxygen species (ROS) as the cellular O₂ tension decreases, O₂-sensitive ion channels, and the mitochondrial electron transport chain (ETC). We have proposed that hypoxia paradoxically stimulates ROS release from the mitochondrial ETC, producing an oxidant signal capable of triggering increases in intracellular Ca²⁺ concentrations during hypoxia. This response appears to be initiated by the release of Ca²⁺ from intracellular stores, followed by the entry of extracellular calcium via L-type Ca²⁺ channels and/or through capacitative calcium entry via store operated calcium channels. The increase in cellular oxidant stress during hypoxia can be detected using ROS-sensitive probes such as dichlorofluorescein or dihydrorhodamine, by novel FRET-based redox sensors or redox-sensitive fluorescent proteins, and by EPR spectroscopy. The primary site of ROS generation during hypoxia appears to be Complex III, although increased oxidant generation at other sites including Complex II may be sufficient to activate the response under some conditions. The exogenous addition or over-expression of antioxidant enzymes abolishes the cytosolic ROS signal and the increase in intracellular calcium in pulmonary artery smooth muscle cells. By contrast, increases in oxidant generation, or administration of exogenous oxidants during normoxia can activate the hypoxia response pathway under normoxic conditions. Much controversy still exists regarding the cellular mechanisms of oxygen sensing. However, there is good agreement that this mechanism plays an important role in survival at stages ranging from development through adulthood.