FRET is the acronym of the Förster (Flourescence) Resonance Energy Transfer. The Förster energy transfer is the phenomenon that an excited donor transfers energy (not an electron) to an acceptor group through a non-radiative process. This process is highly distance-dependent, thus allowing one to probe biological structures. One common application is simply to measure the distance between two positions of interest on a big molecule, generally a biological macromolecule, by attaching appropriate donor-acceptor groups to the big one. If the big molecule only involves one donor and one acceptor group, the distance between the donor and the acceptor can be easily measured if there is no conformational change within this process. Besides, if the molecule has a huge conformational change, one may also measure the dynamical activities between two sites on this macromolecule such as protein interactions. Today, this technique is widely applied in many fields such as single-molecule experiments, molecular motors, biosensors and DNA mechanical movements. FRET is also called the "Spectroscopic Ruler" because of its intrinsic convenience.