In this chapter, we treat the total free energy of the magnetic system as a classical entity, which means that (1) there are no quantum mechanic operators, (2) the magnetization represents a thermodynamic average value over a given domain, and (3) the domain is defined over a region that is greater than the so-called exchange coherence length. Typically, this coherence length is in the order of 50-100 Å in strong ferromagnets or ferrimagnets. The magnetization also implies a net magnetization as in the case of ferrimagnets. Once the free energy of the system is expressed in terms of classical vector variables, all of the mathematical phenomenology developed over the years can be deployed to predict various microwave phenomena from the free energy expression.