This chapter presents the theoretical basis for the application of an elasto-plastic model named Cam-clay used for calculation of strains in a soil element for a given change in the state of stress. Cam-clay incorporates the critical state theory plus a series of plasticity concepts, which is briefly discussed. A computer program named Cris is distributed with the book for hands-on training on critical state models. The program performs simulation of triaxial tests results. The elasto-plastic models differ according to the assumption made for the energy dissipation during the plastic regime. The chapter also discusses the different patterns of stress-strain curves considered in elasto-plastic models. Finally, it presents a summary of critical state parameters for soft clay used as default values in the Cris program.