The electronic and optical properties of nanomaterials are depending on their size, shape, and local environment, demanding theoretical treatments that are distinct from those of bulk solids. I will describe the development of a theory of nonlocal electronic screening, including its application to understand the large band gap renormalization and binding energies of excitons in nanomaterials, including transition metal dichalcogenides and halide perovskites. Within the same framework, a discussion of the exciton fine structure of luminescent perovskite nanocrystals will be presented. Finally, I will describe the development of new and affordable ab initio methods in the GW/BSE framework that are applicable to large molecules and nanomaterials, obviating the need for careful model constructions.