Field-flow fractionation (FFF) is a versatile analytical separation technique that has proven to be applicable to a wide range of polymers, colloids,and fine oarticles over the effective molecular weight range 103 to 1016, corresponding to diameters of 0.001 to 30 µm. Several subtechniques of FFF have been developed for which there are precise theoretical relationships of retention to particle parameters. Fractionation takes place in a thin flow channel by the interaction of a lateral field (gravitational or centrifugal in the case of sedimentation FFF, cross flow in flow FFF, electrical in electrical FFF, and temperature differential in thermal FFF) with the flow profile. Steric FFF, a limiting form of FFF, is applicable to the largest particles, from 1 up to 30 μm or more in diameter, and can also be used in a preparative mode. Altogether FFF has the potential of separating and characterizing radioactive species and the diverse materials with which they are associated in the environment over a size range where analysis by conventional techniques is difficult or impossible.