The theory of ion motion in field asymmetric ion mobility spectrometry (FAIMS) conditions has been developed on the bases of an effective field approach and applied to cylindrical and spherical geometries of FAIMS analyzers. The resulting analytical formulae relate compensation voltage to ion characteristics such as non-linear ion mobility coefficient, ion mobility and ion mass and charge. They permit taking into account ion inertia at small pressure of a buffer gas and at high ion mass.