The extent of strain softening controls strain localization and governs the character of the post-yield response of polymer glasses. In order to understand how solid particles affect strain softening in a common glassy polymer, we investigate the composition dependence of the upper (sigma(y)), lower (sigma(fl)), and rejuvenated (sigma(yr)) yield stresses of neat PMMA as well as glass microparticle (MP) and silica nanoparticle (NP) filled PMMA composites. All yield stresses increase with the filler volume fraction v(f), as expected; the extent of this enhancement increases with T, and is uniformly larger for NP- than for MP-filled systems. This trend cannot be interpreted using simple volume-replacement models; we interpret their breakdown as arising from NP-imposed alteration of segment-scale packing and dynamics at the filler-matrix interface. Furthermore, they vary qualitatively differently with temperature and strain rate, leading to disparate responses of two measures of strain softening (Delta sigma(fl) = sigma(y)-sigma(fl) and Delta sigma(yr) = sigma(y)-sigma(yr)). We analyze these results in terms of recent microscopic and constitutive models. The strain rate dependence of Delta sigma(fl) agrees well with predictions of the Chen-Schweizer PNLE model, while poorer agreement is found for temperature dependence. Finally, we extend the recently developed "three-region" picture of strain softening (van Breemen LCA, Engels TAP, Klompen ETJ, Senden DJA, Govaert LE, J Polym Sci Polym Phys 2012, 50, 1757) to composite systems. Our results should lead to an improved understanding of the factors controlling plastic deformation of polymer composites. (C) 2015 Elsevier Ltd. All rights reserved.