Critical state models are highly successful in reproducing the behavior of soft clays as far as the major differences between elastic and plastic behavior are concerned. However, they are not able to model the non-linear behavior of soils at small strains. The latter is now well understood in terms of empirical evidence, but less satisfactory progress has been made on theoretical modelling. The purpose of this paper is to draw together two diverse topics in soil mechanics: small strain stiffness and critical state modelling. This is achieved within a thermodynamically based formulation of plasticity theory, using the continuous hyperplasticity approach recently developed by Puzrin & Houlsby.