The ‘Förster (or Fluorescence) Resonance Energy Transfer Microscopy’ is a specialized application of fluorescence microscopy. In the FRET effect, energy is transferred from a donor molecule to an acceptor molecule. The efficiency of this energy transfer decreases with the sixths power of the distance and also depends on the orientation of the molecules. Due to this strong distance dependency, one can optically measure distances from 0 to approx. 10 nm.