Generally speaking, the word electrostriction is used to describe electric field-induced strain, and hence frequently also implies the “converse piezoelectric effect.” According to solid-state theory, however, the converse piezoelectric effect is defined as a primary electromechanical coupling effect where the induced strain is directly proportional to the applied electric field, while electrostriction is a second-order coupling in which the strain is proportional to the square of the electric field. Thus, strictly speaking, they should be distinguished. However, because the piezoelectric effect for a ferroelectric, which possesses a centrosymmetric high-temperature prototype phase, originates from the electrostrictive coupling in the phenomenological theory, these two effects are closely related. In a piezoelectric ceramic, the additional strains that develop with the reorientation of ferroelectric domains are also important. A description of ferroelectricity in terms of crystal structure is presented first in this chapter; then, a tensor/matrix description of piezoelectrics is introduced. We review a phenomenological description of the piezoelectric/electrostrictive and magnetostrictive effects in detail. Domain reorientation and loss mechanisms are finally treated in this chapter.