With the increased availability of computers of various sizes, it is becoming more common to predict the responses of geotechnical structures using numerical analyses which incorporate more realistic models of soil behaviour. The main objective of this research is to develop and evaluate a series of unified critical state models. These models are then used to solve some typical boundary value problems in geotechnical engineering. The new models are based on a critical state model called CASM which was formulated based on both the state parameter concept and a non associated flow rule. The main feature of CASM is that a single set of yield and plastic potential functions is used to model the behaviour of clay and sand under both drained and undrained loading conditions. These models are developed by incorporating a new non-linear elasticity rule, the combined hardening concept and the bounding surface plasticity theory. A new non-linear elasticity rule for clay materials is introduced into CASM, this gives a better prediction on the behaviour of soil. The new combined volumetric-deviatoric hardening model is named CASM-d and provides a better prediction of the behaviour of lightly overconsolidated clays and loose sands. The new bounding surface model is named CASM-b and provides a more realistic prediction of soil behaviour inside the state boundary surface. The new cyclic bounding surface model is named CASM-c and provides a good prediction of soil behaviour under cyclic loading conditions. To evaluate their adequacy, CASM and its extensions are implemented into a finite element package called CRISP. This program was specifically developed to incorporate the critical state type of constitutive models. The analyses of a variety of typical geotechnical engineering problems are carried out to further check the validity of the new constitutive models. The models prove themselves to be very robust and useful tools for solving a wide range of practical geotechnical problems under different loading conditions.