The heterodimeric transcription factor HIF (hypoxia-inducible factor), consisting of a labile α-subunit and a stable β-subunit, is a master regulator of genes involved in acute or chronic adaptation to low oxygen. Studies performed over the past 5 years revealed that HIFα-subunits are enzymatically hydroxylated in an oxygen-dependent manner. Hydroxylation of either of two conserved prolyl residues targets HIFα for destruction by a ubiquitin ligase containing the von Hippel–Lindau tumor suppressor protein whereas hydroxylation on a C-terminal asparagine affects HIF transactivation function. Pharmacological manipulation of HIF activity might be beneficial in diseases characterized by abnormal tissue oxygenation including myocardical infarction, cerebrovascular disease, and cancer.