The formulation of a linear theory of piezoelectric materials is the focus of the third part of this book. This serves as the basis for the subsequent physical modelling of the static and dynamic behaviour of piezoelectric bending transducers. The physical concept of energy density is of central importance. This chapter focuses on the derivation of the energy density of elastic deformation. In any material body, external mechanical stresses result in internal mechanical stresses and strains. Their physical nature results naturally from the equilibrium conditions of forces and moments on an elastically deformable volume differential in static equilibrium on the one hand, and from the spatial displacements of the spatial points of the volume differential as a result of external mechanical loads on the other hand. In conjunction with the definition of the work differential, a compact formulation of the energy density of elastic deformation is obtained. The fact that this form of energy related to the volume represents a state function is of particular importance in the context of later thermodynamic considerations.