The performance of a catalytic fluidized-bed reactor is strongly dependent on the properties of the catalyst of which the particle-size distribution is one. The main influences on the particle-size distribution are attrition of the catalyst particles, and the classifying effect of the solids recovery system. In a fluidized-bed reactor a particle will be subjected to attrition due to different mechanisms in different parts of the system, namely attrition by gas jets near the bottom of the fluidized bed, bubble-induced attrition in the fluidized bed itself, and attrition during the passage through a Cyclone. All these different attrition mechanisms are considered in this work by different mathematical models. It is known that a fresh catalyst is much more fragile and exhibits a much higher attrition rate at the beginning of exposure to mechanical stress than under steady-state conditions. Depending on the mechanism the particles need different times or in the case of attrition in a cyclone a certain number of passages to reach a constant value of the attrition rate. In the fluidized-bed system a particle will during its aging experience all the different attrition mechanisms. In order to summarize the effect of these stresses on the particle within the different parts of the fluidized-bed system, the concept of the "stress history" has been developed, which allows a uniform treatment of the different attrition mechanisms. This concept has been implemented into an existing population balance model. Experiments with a FCC-catalyst in a fluidized/bed-cyclone circulation system are well described by this model. (c) 2007 American Institute of Chemical Engineers.