In the present work an approach has been developed to predict the changes in particle size distribution of solids in fluidized bed systems with recirculation of solids. Several effects in fluidized bed systems are influencing the local particle size distribution and have been identified. They are grouped in separation effects, transport effects and degradation effects of the individual particle size. Since a lot of fluidized bed applications are dealing with mixtures of particles of different materials this was also introduced in the scope of the present work.