In this chapter we will discuss the response of matter to external perturbations by electromagnetic fields and point out the properties of matter in the limit of weak and strong fields. If one inserts matter into a magnetic (electric) field, it creates a macroscopic magnetization (polarization). The two following effects play an opposite role: 1. Already existing dipole moments (permanent dipole moments) are aligned by the external field. This leads to the temperature dependent paramagnetism or the corresponding electrical analogue, the orientation polarization (see electrodynamics).