This chapter deals with the way the evolution of barrier properties with time has been predicted, with a focus on repository concepts in clay formations. It considers the stability of clay materials in response to the predicted perturbations arising from how a repository system evolves. The chapter then focuses on recent improvements and perspectives for reactive transport modeling in the framework of studies on predicting how radioactive waste disposal will evolve. This modelling is a very useful tool in assessing the performance and safety of the various radioactive waste disposal concepts. Next, the chapter describes the basic principles that have guided how the reactive transport models are constructed for establishing and evaluating the long‐term geochemical behaviour of repository systems. It further focuses on the generic conceptualization of these calculations, on how materials are described and represented, and on the importance of databases for the outcome of the simulations.