A generalized constitutive model for clays, based upon the concept of the bounding surface in stress space, is developed within the framework of coupled elastoplasticity-viscoplasticity and critical state soil mechanics. The prominent feature of the bounding surface concept is the fact that inelastic deformations can occur for stress points within the bounding surface at a rate depending on the proximity of the actual stress point to a properly defined" image" point on the surface itself. The material state is expressed in terms of the effective stress and the inelastic change in void ratio. The consideration of viscoplastic characteristics sets the current model apart from previous bounding surface formulations for soils and allows for the description of such phenomena as secondary compression, stress relaxation, the change in pore pressure and deviatoric strain under conditions of undrained creep, rate dependent response, etc. Futhermore, the bounding surface is defined in terms of a single ellipse instead of a combination of three distinct surfaces, as was done in the past.