The Hardening Soil Model (HS-model) is formulated in the framework of classical theory of plasticity. In the model the total strains are calculated using a stress-dependent stiffness, different for both virgin loading and un-/reloading. The plastic strains are calculated by introducing a multi-surface yield criterion. Hardening is assumed to be isotropic depending on both the plastic shear and volume strain. For frictional hardening a non-associated and for cap hardening an associated flow rule is assumed. In this paper you will find more background theory of the formulation of the model and on the calibration of the model.