It has long been known from fluorescence recovery after photobleaching experiments that the mobility of most cell surface receptors is much smaller than expected for free diffusion of proteins in a fluid lipid bilayer. Single‐particle tracking experiments are currently revealing the complexity of the constraints to free diffusion. Evidence has been obtained for several different processes: domain‐limited diffusion, temporary confinement and anomalous diffusion. The type of motion exhibited by a given receptor will profoundly influence the rate of any functional process which requires movement in the plane of the membrane. In particular, anomalous diffusion greatly reduces the distance travelled by a receptor on a time scale of minutes.