The deformation within soils is commonly observed to concentrate in narrow zones called shear bands. In the recent years, the localization of strain in geomaterials has been the subject of extensive research. Localization is a physical phenomenon in which the nearly homogenous deformation of the body is abruptly changed into a highly concentrated deformation mode, usually in the form of a single or multiple narrow shear bands. Strain localization is a very significant phenomenon because it is likely to correspond to the actual types of failure in geomechanical engineering practice. The Critical State Models in soil mechanics are widely used in many geotechnical applications involving numerical predictions of stability and deformation behavior of soils such as clay, silty clay and sandy clay. The present work aims to develop these models with a simple, efficient and reliable technique to accurately predict the deformation field, stress distribution, strain softening and shear band formation in soil structures, especially the stability of slopes for the different boundary value problems. Firstly, an attempt is made to review the loading criteria, flow rule and hardening rule in plasticity. Critical State Models have been converted into convenient forms for use in the finite element analysis. An attempt has been made to generalize the Critical State Model in perfectly plastic and strain hardening associative and non-associative flow rule. The hardening behavior of the clay soil always has a higher load-carrying capacity than perfectly plastic behavior. The collapse in both perfectly plastic, strain hardening associative and non-associative flow rule is of approximately the same value. Secondly, the Critical State Model, with additional softening non-associative flow rule has been discussed. This considers the beginning and development of shear bands. The structural softening begins at the ultimate load. The necessary condition for the beginning of localization depends on current values of the stress components. The material softening is related to the reduction of the cohesion and friction angle from the peak to residual values, based on the second invariant of the deviatoric plastic strain. According to this approach, strain softening is interpreted in the constitutive model as a particular type of elastic-plastic behavior with negative hardening. An application of the Critical State strain localization softening model to slopes has been presented. The results of this study show that a numerical analysis accounting for the effects of softening can provide an estimation of the non linear effects. It should be observed, however, that the possible use of this procedure for the assessment of stability of actual slopes would require a more refined characterization of the mechanical parameters. In fact, the gradual concentration of strains into shear bands, which governs the development of failure, is strongly influenced, even by minor local variation of mechanical parameters, such as the residual friction angle or the parameter governing the rate of loss of the mechanical resistance. Key words: Elasto-plastic Critical State Model, perfectly plastic, isotropic strain hardening, associative and non-associative flow rule, strain localization.