In this paper a new constitutive model recently suggested by the authors, referred to as the Sydney Soil model, is introduced. The Sydney Soil Model is formulated within the framework of Critical State Soil Mechanics and is suitable for describing the behaviour of structured soils including both clays and sands. In the proposed model, the mechanical behaviour of soil is divided into two parts, that at a reference state and that attributed to the influence of soil structure. The reference state behaviour is formulated according to the soil properties at the critical state of deformation, based on the concept of plastic volumetric hardening. The effects of structure are captured in the model by incorporation of the additional voids ratio that arises due to the presence of soil structure. The formulation is generalized to include both isotropic compression and general shearing. The paper contains the following two parts: (1) the theoretical framework of the Sydney Soil Model; and (2) the capacity of the model to predict laboratory single element tests, including a summary of the main features of soil behaviour that can be successfully captured by the model.