Oxygen is essential for survival of aerobic organisms. Sensing changes in the environmental oxygen concentration and appropriate adaptation to such changes are essential for organisms to survive. Hypoxia inducible factor 1 (HIF-1) is the key transcription factor in controlling the expression of oxygen-dependent genes required for this adaptation. HIF-1 is a heterodimer of an oxygen dependent alpha-subunit and constitutive beta-subunit. Abundance and activity of HIF-1 is controlled by post-translational hydroxylation. Microscopic analysis of the assembly and activation process of HIF-1 has become an important tool to better understand HIF-1 regulation. Confocal laser microscopy provides exact images of HIF-1alpha that is translocated into the nucleus under hypoxia and its disappearance upon reoxygenation. To exactly follow the protein-protein interaction during the assembly process of HIF-1, both subunits were labeled by fusing them to fluorescent proteins. Fluorescence resonance energy transfer (FRET) was used to determine the interaction of both subunits in living cells by confocal microscopy.