Single-molecule Fluorescence Resonance Energy Transfer (or sm-FRET) is a biophysical technique used to measure distances at the 1-10 nanometer scale in single molecules, typically biomolecules. It is an application of FRET ("ensemble FRET") wherein a single donor and acceptor FRET pair are excited and detected. One benefit of studying distances in single molecules (rather than in ensembles) is that heterogeneous or dynamic biological processes can be studied without averaging.