Forster/Fluorescence Resonance Energy Transfer (FRET) is a physical phenomenon which involves radiation-less energy transfer (via long-range dipole-dipole interaction) from excited donor fluorochrome to another molecule or acceptor. FRET relies on distance-dependent energy transfer (donor and acceptor) and can only occur when the distance between donor and acceptor is less than 10 nm. Due to sensitivity to distance, FRET is used in studying molecular interactions. This chapter aims to discuss the concept and applications of FRET in a cryptic way.