The need for particle trajectory determination within field-flow fractionation and SPLITT fractionation channels is discussed. In the case of steric or steric/hyperlayer FFF, the trajectory followed by a particle determines its elution time. With hydrodynamic relaxation, the determination of a representative set of trajectories can also provide information on the band spreading inherent to the technique. In the simplest case of SPLITT fractionation where a binary separation is the objective, the trajectories determine the point at which a separative cut is made, as well as the sharpness of the cut. The influence of hydrodynamic lift forces, applied transverse and longitudinal fields, and secondary crossflow on particle migration are discussed, as well as the perturbing influence of the presence of the channel walls. Finally, a computer program is used to generate sets of particle trajectories to illustrate certain aspects of the preceding discussion.