A model is presented for the entrapment of a distribution of particles by a distribution of pores. The fraction of particles retained and their distribution of sizes can be calculated for evolving and static porous structures. For evolving structures the change in distribution and fractional retention through the structure can be calculated. Also, the variance in these parameters between zones in the plane of the sheet can be calculated. The agreement between the theory and experimental data is good . The theory has relevance to the retention of fillers and fines in the papermaking process and to more general problems of stochastic porous media.