The bioluminescence resonance energy transfer (BRET) methodology allows for the study of protein-protein interactions as well as conformational changes within proteins or molecular complexes. BRET is a highly versatile technique that can be applied to in vitro studies using purified proteins, crude cell membranes, cell fractions obtained by centrifugation on a density gradient, as well as permeabilized cells. Importantly, BRET also allows for monitoring of protein-protein interactions, in real time, in intact living cells that can be submitted to various stimuli. Moreover, quantitative BRET analysis also permits a pharmacological approach of protein-protein interactions, allowing one to determine whether a given stimulus induces a conformational change within preassociated partners or increases the association (recruitment) between two separated partners. Determination of the proportion of the dimeric vs monomeric form of a protein in the cell also is possible. Therefore, the BRET technology can be considered as a new and powerful tool in the field of protein-protein interactions.