  The experimental literature on the motion of mesoscopic probe particles
through polymer solutions is systematically reviewed. The primary focus is the
study of diffusive motion of small probe particles. Comparison is made with
measurements of solution viscosities. A coherent description was obtained,
namely that the probe diffusion coefficient generally depends on polymer
concentration as D_p = D_0 exp(-alpha c^nu). One finds that alpha depends on
polymer molecular weights as alpha ~ M^0.8, and \nu appears to have large-M and
small-M values with a crossover linking them. The probe diffusion coefficient
does not simply track the solution viscosity; instead, D_p eta typically
increases markedly with increasing polymer concentration and molecular weight.
In some systems, e.g., hydroxypropylcellulose:water, the observed probe spectra
are bi- or tri-modal. Extended analysis of the full probe phenomenology implies
that hydroxypropylcellulose solutions are characterized by a single,
concentration-independent, length scale that is approximately the size of a
polymer coil. In a very few systems, one sees re-entrant or
low-concentration-plateau behaviors of uncertain interpretation; from their
rarity, these behaviors are reasonably interpreted as corresponding to specific
chemical effects. True microrheological studies examining the motion of
mesoscopic particles under the influence of a known external force are also
examined. Viscosity from true microrheological measurements is in many cases
substantially smaller than the viscosity measured with a macroscopic
instrument.
