Because of the increasing use of dielectrophoresis in the dielectric characterization and sorting of living cells or their parts, it has become important to establish carefully the theoretical backgrounds for this effect. A comparison with experiment is made of the several versions of the theory for the dielectrophoretic force exerted by nonuniform electric fields upon a neutral object. The three fundamental approaches: the Maxwell-Strattonstress tensor, the effective dipole moment, and the âHelmholtzâ energy approach are presented along with the general solution given earlier by Pohl and Crane. These are found to agree closely with experiment in predicting the dielectrophoretic force upon various rods hung in specially shaped (isomotive) field distributions. On the other hand, an alternative formulation based upon a debatable assignment of fields local to the dipoles gave a good fit to the experimental data only for materials of very low permittivity, and fitted poorly in the case of highly polarizable materials. An improved derivation of the theory for stable dielectrophoretic levitation is also presented. This phenomenon is of particular interest in that it is based upon an apparent violation of the Earnshaw's theorem, and is useful in the study of the dielectric properties of individual living cells.