Genuine, physics-based understanding of initiation phenomena in plasticbonded explosives (PBXs) requires knowledge of the physics and chemistry at mesoscopic scales that are far larger than can be simulated directly using atomistic detail, yet far smaller than is directly resolvable in practical engineering scale continuum simulations. Initiation is determined by localization phenomena that arise due to the heterogeneous character of most explosive formulations. Indeed, the "average" temperature behind a weak shock is not a useful measure for understanding initiation phenomena; rather, it is the tails of the distributions in temperature, stress, and strain rates, localized to small, spatially distributed volumes in the material (hot spots), that dictate the outcome of a given loading event. Important factors for predicting hot spot formation and subsequent extinction or growth/coalescence include particle size, concentration, morphology, and void content; physical and chemical interactions between grains and binder; thermophysical and mechanical properties of the constituents and interfaces between them; and, of course, the inherent chemical stability of the explosive component(s) in the formulation. We are in the process of computing many of the thermophysical and mechanical properties required for a complete specification of constituent models for use in mesoscale simulations, wherein grains and binder in representative volumes of a PBX are spatially resolved and then studied within a continuum hydrodynamic framework. In addition to calculating specific properties of interest, we have recently undertaken a series of large-scale molecular dynamics simulations of energetic crystals to understand dissipation phenomena in dynamically loaded single- or poly-crystalline samples; for instance, plastic deformation and stress/energy localization mechanisms, phase transitions, and so on. Recent and ongoing work in these areas will be discussed, along with their specific relevance to emerging mesoscale simulation capabilities.