The concept of multilevel dynamical models, developed by using the method of volume averaging, is presented for polydisperse systems under conditions of phase transitions. Equations of a two-level model are discussed in detail, and multilevel models are obtained as generalization of those. A consistent definition of the population density function is given the governing equation of which may also be derived in the framework of the present theory. Two- and multilevel models appear to be of special importance in dynamic analysis of polydisperse systems. These models are expected to exhibit complex dynamic behaviour due to the action of lower levels, yielding stabilizing or destabilizing effects.