Magneto-dielectric materials include materials that exhibit both electrical and magnetic properties. Plastics and wood materials may be included if one considers diamagnetism as a special magnetic property. There is no interest in the microwave properties of such materials in this book. So, let us be more focused about what we consider interesting in exploring the microwave properties of magneto-dielectric materials. Magnetic ﬁlms, in general, may be considered to be both magnetic as well as dielectric materials. The point is that in order to describe the microwave properties of magnetic materials completely one must include electrical, elastic, and magnetic properties of the material simultaneously. They are inseparable, because they are all coupled to each other. We start ﬁrst with the thermodynamic properties of electric dipole moments and then derive a free energy appropriate for electrical, elastic, ferroelectric, and superconducting materials.