Fluorescence resonance energy transfer (FRET) operates on the principle of nonradiative energy transfer between a donor and an acceptor fluorophore when they are in close proximity (typically 1–10 nm). It is a valuable method for identifying protein–protein interactions in live cells, where not only the physiological condition but also the exact spatial information where interactions occur can be obtained. FRET offers significant advantages for studying molecular interactions and dynamics during viral infections, including high sensitivity and specificity, real-time monitoring capabilities, and versatility in experimental design. In this chapter, we outline the application procedure and provide troubleshooting notes for fluorescent protein-based FRET analysis, including single-molecule FRET (smFRET), when conducted in virological studies.