FRET stands for Forester Resonance Energy Transfer. That doesn’t say a lot so we can break things down. A protein system that uses FRET takes advantage of fluorescent proteins. A fluorescent protein can be excited at a certain wavelength of light by a laser and emit an intensity which is absorbed by a detector. Based on the intensity of light detected you can quantify the concentration of fluorescent protein. Using two fluorophores, energy transfer occurs as the FRET donor is excited and transfers energy for the acceptor to emit. The emission intensity of the acceptor detected can help us quantify the concentration of a ligand binding to a protein. Think about this. If we have two fluorophores on a protein that come together to create light as a ligand binds the protein (protein changes conformation) then the intensity of light is directly correlated to ligand concentration! Some people also use fluorescent proteins to not only detect binding events but determine the localization of proteins in cells!