and co-workers[@gray1974a; @turner1976a], who in 1974 and 1976 observed the diffusion of polystyrene spheres through hyaluronic acid and dextran solutions, and compared probe diffusion with the rheological properties of their solutions. The subsequent four decades have seen an enormous extension of this approach[@phillies2011a], including studies of probes in highly viscous simple liquids[@phillies1981z], polymer melts[@lin1986a], chemically cross-linked gels[@schmidt1989b], surfactant solutions[@phillies1993b], protein solutions[@ullmann1985a], and the interior of living cells[@lubyphelps1987a]. More recently, quasielastic x-ray scattering has been used to extend the range of distance scales over which diffusion can be observed[@dierker1995a].

Probe diffusion has also been studied by a series of other physical techniques, each technique being sensitive to distinctive range of time and distance scales or other features of probe motion. For example, fluorescence correlation spectroscopy[@magde1972a], which by varying the probe concentration can measure both the self diffusion coefficient and the mutual diffusion coefficient of the labeled species[@phillies1975a; @scalettar1989a], has in recent years been extensively used to measure tracer diffusion. Recent work using probe diffusion is sometimes termed *microrheology*, the term microrheology referring to a particular model[@mason1996a] for interpreting quasielastic scattering spectra. In some studies, probe diffusion has been viewed as being of interest because it is a path to measuring the 