Computational methodologies that can be used to accurately calculate spectroscopic properties are naturally based on electronic-structure methods. However, the application of such methods is limited to relatively small molecular systems due to the high computational cost and the unfavorable scaling with the size of a system. Fortunately, many spectroscopic processes are spatially localized, and therefore only a small part needs a quantum mechanical (QM) description. Still, the environment surrounding the central part cannot be neglected because it plays a crucial role, both directly through electrostatic interactions and indirectly by restricting the geometry. A commonly employed approach that takes advantage of the locality is quantum mechanics/molecular mechanics (QM/MM), which combines a quantum description of the central subsystem with a classical description of the environment using a molecular mechanics (MM) force field. The direct environmental effects on the central quantum subsystem can be included by adding an embedding potential to the Hamiltonian of the quantum subsystem.