ElectromagnetismElectromagnetism is a very broad and profound subject. Frankly, it is not an easy task not only for a non-expert but also even for a physicist to completely understand it. This chapter provides a minimal idea of electromagnetism for non-physicists who are trying to theoretically compute vibrational spectra according to the quantum/classical mixed approach. Note, therefore, that this chapter does not cover electromagnetism as a whole. Instead, this chapter topically deals with selected equations and physical quantities that are the most necessary to carry out the theoretical calculations of the IR, Raman, and sum frequency generation spectra of molecular systems according to Chaps. 2 and 3 .