The hypoxia-inducible factor-1 (HIF-1) is a basic-helix-loop-helix (bHLH) heterodimeric transcription factor activated by reductions in oxygen concentration (hypoxia). Activated HIF-1 upregulates expression of genes involved in the adaptation of higher organisms to hypoxic conditions, caused by e.g. high altitude, anemia, wound healing or during development. Examples of these oxy-genes include erythropoietin, a hormone regulating erythropoiesis and hence the oxygen transport capacity, and vascular endothelial growth factor, a potent inducer of angiogenesis leading to increased blood capillary density. The HIF-1 heterodimer is composed of a HIF-1alpha and an ARNT subunit, both belonging to the explosively growing PAS subfamily of bHLH transcription factors. Closely related, but differentially expressed, factors have recently been cloned, at least one of which can also be activated by hypoxia. In this review, we present a survey of the bH LH-PAS family as well as of the genes regulated by HIF-1, and we summarize our current knowledge on the oxygen-dependent activation of this fascinating transcription factor.