The customers of the aggregate producing industry want a reduction of misplaced material, which puts heavy demands upon the suppliers. Misplaced material consists of particles below the separation limit remaining in the coarse fraction and coarse particles in the fine fraction. To minimise these problems, it is necessary to have a fundamental understanding of the screening process. There are two simultaneous processes to consider during screening, stratification and passage of particles through the apertures in the screen surface. The purpose of this article is to provide a physical explanation showing how the rate of stratification and the rate of passage vary along the screen. The interrelation of the two processes is described in the flow model presented here. The rare of stratification varies with the proportion of fine material and the ratio of the sizes of the particles. The rate of passage depends on the probability that the particles will pass through the apertures and on the amount of fine material on the screen surface. When the whole screen surface is covered with fine material, the rate of passage reaches a maximum. The model has been implemented in a calculation program. The general outcome of the calculations is in accordance with experiments.