Signalling through the epidermal growth factor receptor (EGFR) is preceded by its dimerization, which has typically been thought to occur through a ligand-induced conformational change. Here, the dimerization dynamics of individual EGFR molecules have been determined in living cells in real time, using a quantum-dot-based approach. Unliganded EGFR molecules undergo spontaneous and reversible dimerization; these pre-formed dimers are primed for ligand binding and signalling and are enriched at the cell periphery