The absorption of light by matter follows a universal mechanism that drives crucial reactions in both natural and engineered systems. Processes ranging from photosynthesis, to photocatalysis, to photovoltaic energy harvesting all begin the same way: the absorption of light creates an exciton—a correlated electron-hole pair that carries energy rather than charge. The dynamics of excitons are key to understanding these processes, but quantitative predictions of exciton dynamics far exceed current theoretical capabilities, leaving fundamental questions unanswered. I will develop a range of novel first-principles theories to enable computation of the life cycle of excitons in their totality. This will create a comprehensive understanding of light-matter interactions, revealing the fundamental science of excitons at all energies and timescales, and paving the way for a new generation of technological developments in optoelectronics, energy harvesting and transport, and quantum information.