Multiscale simulations have become important tools in many fields of chemistry, physics, and biology. In particular, quantum mechanics / molecular mechanics (QM/MM) approaches are widely used to investigate large and complex molecular systems where quantum effects of a subdomain play a key role. Examples include (bio)chemical reactions and light-matter interactions that require explicit modeling of the electronic structure of the active parts of the total system. The rest of the system is modeled using a molecular mechanics description, in which the effect of changes in the electronic structure is taken into account via an effective potential depending only on the atomic positions. However, depending on the physical or chemical phenomena being studied and the level of accuracy that is needed, different levels of approximation may be employed, ranging from a fully quantum mechanical (QM) description, over atomistic molecular mechanics (MM) and coarse-grained (CG) modeling, to continuum mechanics (CM). Multiscale approaches take advantage of the fact that it is often sufficient to use the more fine-grained and accurate models on a smaller subsystem of a complete molecular system while the remainder can be modeled using coarser, less accurate, but computationally efficient models.