Field–Flow Fractionation (FFF) is a relatively new technique for characterizing colloidal materials and macromolecules. It is an elution technique and the retention volume under selected experimental conditions is immediately related to the number and weight average particle diameter or to the particle mass. The accuracy in the determination of the above parameters for a given colloid or macromolecule is dependent on a number of experimental parameters among which the surface potential of the particles, the nature, pH and ionic strength of the suspending medium, the effective Hamaker constant, the concentration and pretreatment of the sample, as well as, the relaxation time and the applied field strength.