Based on these promising results in single molecules or dimers, it is natural to turn the attention to more complex material systems, such as solvated dyes, molecular aggregates, thin films, interfaces, or macromolecules. However, as for all other ab initio methods, the system sizes required to realistically and representatively study electronic excitations in such materials exceed by far what is computationally tractable. Yet, the realization that many of such material systems are characterized by a significant amount of disorder leading to -in broad terms -localization of electronic states (in contrast to the Bloch states in hard condensed matter), offers a way out of this conundrum. That is, partitioning the overall system into a region of interest ("active region"), say a dye molecule, which is embedded in a surrounding region such as one containing a solvent. Both regions can then be described at different levels of theory such that the very accurate and computationally expensive method is restricted to the active region, while a less expensive lower-level method is used for the environment. This lower level can be either a simplified quantum mechanical (QM) model in a QM/QM setting , a molecular mechanics (MM) approach (QM/MM) , or a continuum model representing the environment as a structure-less material having realistic macroscopic dielectric properties.