Knowledge of how and when proteins interact in living cells is fundamental to our understanding of cellular biology, and bioluminescence resonance energy transfer (BRET) provides an increasingly popular mechanism for studying these interactions in real time. The technique utilises heterologously expressed fusion proteins linking a bioluminescent donor or complementary acceptor fluorophore to proteins of interest. Resonance energy transfer between these fusion proteins is then detected when they are in close proximity, indicative of association either directly or as part of a complex. BRET is particularly useful for real-time monitoring of ligand-modulated interactions as dynamic changes in protein complex assembly can be observed in a live cell environment.