The discovery of quantum theory has led to explanations for nearly all physical phenomena from the smallest to largest length scales. In recent decades, as quantum theories have become better understood, scientists and engineers now seek to apply the unique advantages of quantum systems to solve practical problems. The purpose of this dissertation is to engineer quantum light-matter systems, or quantum optical matter, for practical applications by using theoretical and computational methods at the intersection of quantum chemistry and quantum optics. This dissertation consists of three parts. In the first, we design and control a class of quantum optical matter, defects in solid-state materials, that can be used as a nanoscale interface between quantum light and quantum matter degrees of freedom. In the second part, we click together simple and generic systems of quantum optical matter, where the particular features of specific systems like defects in solid-state materials have been abstracted away, to form more complex composite systems. We then show that certain composite systems can emit entangled photons and perform multi-qubit gates on photons with applications in photonic quantum computing. Finally, in the third part, we study another application of quantum optical matter--chemistry. In particular, we propose an explanation for experiments in the field of vibrational polariton chemistry where it has been observed that molecules packed into a cavity with fundamental modes resonant with molecular vibrational resonances exhibit altered chemical reactivity.