Both the direct and reciprocal piezoelectric effects are used in many modern sensor and actuator systems. The continuous development of a large number of industrially producible crystalline and ceramic piezoelectric materials has contributed significantly to this. Of particular importance is the ferroelectric oxide ceramic lead zirconate titanate (PZT), which is classified as a functional ceramic. Starting with a brief history of the discovery of piezoelectric ceramics, this chapter focuses on the crystal structure of PZT. From this, the ferroelectric domains on the microscopic scale and the associated domain processes can be clearly explained. The behaviour on the microscopic scale ultimately forms the basis for understanding the macroscopic properties of a PZT ceramic body. In addition to the behaviour under purely electrical and purely mechanical load types, the behaviour under coupled electromechanical loads, to which actuator applications are always exposed in reality, is also investigated in more detail.