Preferential concentration describes the accumulation of dense particles within specific regions of the instantaneous turbulence field. This phenomenon occurs in dilute particle-laden flows with particle time constants of the same order as an appropriately chosen turbulence time scale. The mechanisms which drive preferential concentration are centrifuging of particles away from vortex cores and accumulation of particles in convergence zones. Experimental and numerical studies are reviewed which demonstrate that preferential concentration occurs in a wide range of flows including plane and axisymmetric free shear flows, wall-bounded flows, homogeneous turbulent flows, and complex shear flows. The same basic mechanisms are active in all these flows but the specific effects and the definition of an appropriate time scale change from problem to problem. Preferential concentration has been shown to have a significant effect on droplet combustion, aerosol particle settling and turbulence modification by particles and is expected to be significant in numerous other applications. Current models describing preferential concentration and its global effects on the flow are inadequate.