Hypoxia-inducible factor 1 (HIF-1) is a basic helix-loop-helix-PAS domain transcription factor that is expressed in all metazoan organisms and is composed of HIF-1α and HIF-1β subunits. Under hypoxic conditions, HIF-1 regulates the transcription of hundreds of genes in a cell type–specific manner. The HIF-1α subunit is regulated by O₂-dependent hydroxylation of proline residue 402, 564, or both, by prolyl hydroxylase domain protein 2 (PHD2), which promotes binding of the von Hippel-Lindau protein (VHL), leading to ubiquitination and proteasomal degradation; and O₂-dependent hydroxylation of asparagine residue 803 by factor inhibiting HIF-1 (FIH-1), which blocks the binding of the 300-kilodalton coactivator protein (p300) and CREB binding protein (CBP). The hydroxylation reactions, which utilize O₂ and α-ketoglutarate as substrates and generate CO₂ and succinate as by-products, provide a mechanism by which changes in cellular oxygenation are transduced to the nucleus as changes in HIF-1 activity. Hydroxylase activity is inhibited in the presence of low concentrations of O₂, high concentrations of tricarboxylic acid cycle intermediates (isocitrate, oxaloacetate, succinate, or fumarate), or chelators of Fe(II). Receptor for activated C kinase 1 (RACK1) competes with heat shock protein 90 (HSP90) for binding to HIF-1α and mediates O₂-independent ubiquitination and proteasomal degradation. A growing number of proteins and small molecules have been identified that regulate HIF-1 activity by modulating the physical or functional interaction of PHD2, VHL, FIH-1, RACK1, or HSP90 with HIF-1α.