Recent advances in imaging assays based on bioluminescence resonance energy transfer (BRET) have made it possible to study protein/protein interactions in living cells under physiological conditions. Here we describe protocols for these assays including relevant positive and negative controls, and we also show how they can be combined with protein complementation assays such as bimolecular fluorescence complementation (BiFC) to study three- and four-partner interactions. We also describe a BRET assay that uses SNAP-tagged proteins as a fluorescence acceptor molecule for the bioluminescent donor.