In many classes of glassy polymers, much of the macroscopic response of the material is controlled by crazing. When crazing occurs under well-controlled conditions, as in high impact polystyrene (HIPS) blends, it provides a mechanism of inelastic deformation improving the material toughness. Crazes, however, are also the precursors of cracks and, ultimately, failure. Micromechanical features of the blend, such as particle size, compliance, and volume fraction, must be accurately tailored in order to attain the desired effects. In this work we present a micromechanical model for particle-toughened polystyrene (PS). The finite element model considers a representative volume element (RVE) of the two-phase material and includes special craze elements with nucleation and growth criteria based on experimental observations of craze behavior in PS. The model enables the investigation of the effects of various parameters, such as the size, volume fraction and properties of the second-phase particles, on the inelastic behavior of toughened polymers, with particular regard to craze initiation and growth. Here, we demonstrate its utility by exploring the progression of multiple crazing in a representative HIPS system.