The use of ab-initio quantum-mechanical methods allows for the calculation of a wide range of molecular properties with high accuracy. A significant drawback of such methods is, however, the steep scaling with respect to the size of the system. This is unfortunate considering that most chemical processes occur in solution or other matrices, such as a protein or cell membrane. In response to this, numerous environment models have been developed. Such models rely on the fact that many molecular properties are often localized to a confined area, making it feasible to treat only a small part, the quantum region, with an accurate quantum-mechanical method. The effects from the rest of the system, the environment, on the quantum region are modeled with an approximate but more efficient approach. The quantum region is thus embedded in an environment which is described at various resolutions depending on the model. For example, the environment can be described as a structure-less continuum, such as is done in the continuum solvent models, whereas the more advanced models include a proper quantum description of the environment, such as frozen density em-bedding. Continuum models present a very efficient description of solvation effects of many simple solvents, defined only by their dielectric constant, refractive index, and atomic radii, but are unable to take into account specific interactions, such as hydrogen bonds. Moreover, using continuum models to address heterogeneous and anisotropic environments, such as proteins, is problematic. Models such as Frozen density embedding retains a quantum description of the environment and can address complex environments more accurately, but such models are associated with significantly larger computational cost and complexity. At an intermediate level, there are the hybrid quantum-classical models, such as quantum mechanics / molecular mechanics (QM/MM), in which the atomistic environment is retained but described classically, typically by a molecular mechanics force field.