With the energy densities of elastic deformation and the electrostatic field, two physical quantities were derived in the previous two chapters which are of great importance for the explanations in this chapter. The physical description of elastic dielectrics with piezoelectric properties is closely related to a deeper understanding of the coupled thermodynamic, electrical and mechanical behaviour of matter at the macroscopic level. Starting with an introduction to the most important basic thermodynamic concepts, the first law of thermodynamics is used to illustrate and explore the electromechanical behaviour of a solid in the context of thermodynamics from a different perspective. This type of approach finally allows the derivation of the thermodynamic potentials corresponding to the application and the resulting piezoelectric equations of state with the associated material coefficients. Based on the material coefficient matrices of any anisotropic material (triclinic, no centre of symmetry), the corresponding material coefficient matrices for the lead zirconate-lead titanate system PZT are presented at the end of the chapter.