Ligand binding to cell surface receptors activates signaling pathways in normal and pathologic conditions, and internalized ligand-receptor complexes may continue to signal from endosomes. Accessibility of cell surface receptors and the central function of ligand-receptor binding in signal transduction make ligand binding a prime target for therapeutic agents. We describe a Gaussia luciferase complementation method for imaging ligand-receptor binding in cell-based assays and living mice. While we illustrate this imaging method for chemokine ligand CXCL12 and its receptors CXCR4 and CXCR7, this imaging strategy can be generalized to a large number of ligand-receptor interactions.