Abstract: Bioluminescence resonance energy transfer (BRET) biosensors for assessing the intracellular localization, internalization and trafficking into cellular compartments of proteins such as receptors, and other biomolecules such as second messengers, are disclosed. These biosensors, which are dependent on the concentration/density of the BRET donor and acceptor in cellular compartments rather that specific protein-protein interactions, use a Renilla GFP/Luc BRET pair, which allows the robust and reproducible monitoring of protein trafficking/localization, with a sensitivity compatible with high-throughput screening (HTS). The use of these biosensors for various applications, including assessing/monitoring protein endocytosis, recycling and intra-cellular trafficking, receptor maturation/rescue by pharmacological chaperones, various endocytosis/exocytosis processes, activation/inhibition, as well as biomolecule concentration/density in different cellular compartments, is also disclosed.