We use coarse-grained computer simulations to delineate the mechanisms governing the steady-shear rheology of polymer-nanoparticle composites. Our studies specifically focus on the regimes where the particle sizes and the interparticle distances become comparable to the polymer sizes and where the interactions between polymer and particles become relevant in influencing the dynamical characteristics. Our results suggest the shear theology of the composite is very similar to that of colloidal suspensions in a simple fluid when polymer theology, the particle-induced changes in the polymer theology and the polymer slip effects are accounted. At dilute and semidilute nanoparticle concentrations, the composite shear theology is shown to be dominated by the shear thinning of the polymer chains which in turn is modified by the presence of the particles. For higher particle loads, the polymeric contribution to the theology becomes much less important and the shear theology is dominated by the particles stresses. Using our results we suggest how a simple empirical model can be constructed for the shear theology of the composite over the entire range of volume fractions. Our results and mechanistic explanations are in very good agreement with associated experimental observations. (c) 2006 The Society of Rheology.