Hypoxia‐inducible factor (HIF) is central in coordinating many of the transcriptional adaptations to hypoxia. Composed of a heterodimer of α and β subunits, the α subunit is rapidly degraded in normoxia, leading to inactivation of the hypoxic response. Many models for a molecular oxygen sensor regulating this system have been proposed, but an important finding has been the ability to mimic hypoxia by chelation or substitution of iron. A key insight has been the recognition that HIF‐ α is targeted for degradation by the ubiquitin‐proteasome pathway through binding to the von Hippel‐Lindau tumour suppressor protein (pVHL), which forms the recognition component of an E3 ubiquitin ligase complex leading to ubiquitylation of HIF‐ α. Importantly, the classical features of regulation by iron and oxygen availability are reflected in regulation of the HIF‐ α/p VHL interaction. It has recently been shown that HIF‐ α undergoes an iron‐ and oxygen‐dependent modification before it can interact with pVHL, and that this results in hydroxylation of at least one prolyl residue (HIF‐1 α, Pro 564). This modification is catalysed by an enzyme termed HIF‐prolyl hydroxylase (HIF‐PH), and compatible with all previously described prolyl‐4‐hydroxylases HIF‐PH also requires 2‐oxoglutarate as a cosubstrate. The key position of this hydroxylation in the degradation pathway of HIF‐ α, together with its requirement for molecular dioxygen as a co‐substrate, provides the potential for HIF‐PH to function directly as a cellular oxygen sensor. However, the ability of these enzyme(s) to account for the full range of physiological regulation displayed by the HIF system remains to be defined.