Piezoelectric ceramics generate strain through the intrinsic piezoelectric effect, the motion of ferroelectric domain walls, or through field-induced phase transitions. The enhanced piezoelectric properties observed in morphotropic solid solutions arise from several distinct, but interrelated, mechanisms associated with the near degeneration of the energy surface from cubic to spherical symmetry. The phenomenological theory of ferroelectricity is used to explain the thermodynamic origins of strain generation mechanisms in these solid solutions. The displacement of ferroelectric domain walls is an extrinsic contribution to the piezoelectric response that can be controlled by modifying the host material with small concentrations of dopants. The concept of “hardening” is introduced; hardening can be useful in applications where piezoelectric energy conversion and low energy loss are more important than large strain. The operative mechanisms of strain generation and energy conversion in technologically important lead-based and lead-free piezoelectric materials are summarized.