The maintenance of oxygen homeostasis is required both in physiological development and tumour growth. Hypoxia inducible factor (HIF) plays a central role in both processes. Reliable methods for visualising HIF a subunits have established that HIF activation occurs in the majority of common cancers. This occurs both by genetic mechanisms and through microenviromental hypoxia. Activation of the HIF pathway has important effects on patterns of gene expression in tumours.