Recent developments in spatially resolved non-linear spectroscopies have offered novel probes of excited-state dynamics on the 100 nm length scale in a variety of molecular materials. Mechanistic interpretation of these measurements, however, is often obscured by material heterogeneities, electronic delocalization, and spectral congestion. While simulation and modeling provide a powerful approach to uncovering the essential dynamics underlying spectroscopic observation, most computational methods remain intractable for mesoscale molecular materials where the number of molecules is massive and the material parameters tend to fall into the broad “intermediate regime” where perturbative techniques breakdown. In this talk, I will discuss recent developments in my group that have enabled the first formally-exact mesoscale simulation of exciton dynamics and spectroscopy in molecular materials. These developments offer a glimpse of a new class of quantum dynamics methods that can illuminate photophysical mechanisms of realistic molecular materials.