Preliminary models for predicting the performance of exposed concrete show that the porosity sub-model plays a pivotal role in linking properties with the developing gradients of cement paste microstructure that arise from drying and carbonation. The difficulties of porosity characterization are reviewed and some alternatives to traditional methods are considered. Property/porosity relationships are considered with particular reference to the volume, size-distribution and continuity of the larger pores. The factors influencing porosity under normal exposure conditions, such as cement hydration, drying and carbonation, are reviewed and the possibilities of modeling their effects are considered. Some comparisons between predicted and measured values of porosity are given. It is concluded that progress in modeling the performance of exposed concrete is likely to be linked with improvements in methods of pore structure characterization.