Bimolecular fluorescence complementation (BiFC) is a powerful technique used to visualize protein–protein interactions (PPIs) in living cells. By reconstituting a fluorescent protein from two nonfluorescent fragments fused to proteins of interest, BiFC enables noninvasive observation of protein interactions within the cellular environment. This method has been successfully applied to study a variety of biological systems, including the interactions of viral proteins with host factors, thereby enhancing our understanding of viral pathogenesis and developing targeted therapeutic strategies. In this chapter, we outline the application procedure and provide troubleshooting notes for BiFC when conducted in virological studies.