Mesoscale models are used to study dynamic deformation and failure in silicon carbide (SiC) and aluminum oxynitride (AlON) polycrystals. Elastic and anisotropic elastic-plastic crystal models represent mechanical behavior of SiC and AlON and grains, respectively. Cohesive zone models represent intergranular fracture. Failure data that can be used to inform macroscopic continuum models of ceramic behavior are collected and analyzed. Studied are effects of grain morphology, specimen size, and applied stress state on behavior of polycrystalline aggregates loaded dynamically at applied strain rates on the order of loVs. Results for SiC demonstrate shear-induced dilatation, increasing shear strength with increasing confinement or pressure, increasing strength with decreasing specimen size (in terms of number of grains), and decreasing strength variability with decreasing size. Results for AlON demonstrate increased initiation of slip activity - particularly in the vicinity of constrained grain boundaries - with confinement.