The study of ligand binding to GPCRs and the monitoring of cellular responses to these external stimuli remains a prime focus for medical research. This paper describes a highly sensitive BRET (Bioluminescence Resonance Energy Transfer) assay (NanoBRET) that allows the monitoring of ligand binding to GPCRs in live cells. The method relies on expression of a newly developed, low molecular weight, Nanoluc luciferase on the N terminus of a GPCR in live cell lines. Monitoring of ligand binding and competitive inhibition studies were efficiently achieved by utilising a fluorescently labelled agonist of the receptor. Ligand binding data and competition assays for the b2 adrenergic receptor will be presented.