The purpose of this work is to simulate shear rejuvenation and aging effects in shear thinning yield stress fluids in a typical rotational rheometer and to provide a common framework to describe the behavior of yield stress materials in general. This is particularly important in the determination of material constants under both steady and unsteady conditions. The breakdown and buildup of structure are studied using a theory based on the Herschel-Bulkley flow model that it is consistent with experimental data. The theory is implemented using a novel computational method. Interestingly, the simulations reveal the existence of time-dependent shear banding that occurs within the gap when the macroscopically imposed shear rate is below a certain critical value. Shear banding is analyzed in detail and results showing the effects of major parameters on the phenomenon are presented. (C) 2009 Elsevier B.V. All rights reserved.