Electronic excitations and their time dynamics are at the foundation of how we use and probe matter. Recent experimental advances allow us to do so with unprecedented accuracy and time resolution, however, their interpretation relies on solid theoretical understanding. This can be provided by first-principles theoretical-spectroscopy, based on many-body perturbation theory and time-dependent density functional theory. In this talk, I will illustrate recent successful examples for how these electronic-structure methods lead to deep understanding, e.g. of light absorption of organo-metal halides and the enhancement of defect diffusion by hot electrons under radiation conditions. While these first-principles simulation techniques allow for predictive accuracy and excellent agreement with experiment, they rely on approximations, and I will illustrate our recent efforts of developing better theoretical understanding of dielectric screening physics and how to bridge time scales from ultrafast electron dynamics to atomic diffusion. Finally, I will describe how incorporating online databases into computational research on excited electronic states can side-step the problem of high computational cost to facilitate materials design.