The response of solids to incident electromagnetic fields is often heuristically formulated in terms of polarization and magnetization fields. The “modern theory of polarization,” and its extension to magnetization, gives this a new level of rigor for time independent applied fields and for macroscopic polarization and magnetization fields. Here we report on a strategy for introducing microscopic polarization and magnetization fields in the presence of electromagnetic fields varying arbitrarily in space and time, and connect this with earlier work on atoms and molecules, identifying important similarities and differences.