The piezoelectric actuator (PEA) is a well-known device for managing extremely small displacements in the range from 10 pm to 100 /spl mu/m. When developing a control system for a piezo-actuated positioning mechanism, the actuator dynamics have to be taken into account. An electromechanical piezo model, based on physical principles, is presented in this paper. In this model, a first-order differential equation is adopted to describe the hysteresis effect, and a partial differential equation is used to describe the mechanical behavior. Since, in practice, a PEA is most often used as an actuator for positioning mechanisms, we considered the influence of such a mechanism on the overall mechanical behavior of PEA and positioning mechanism together. For a well-designed mechanism, the overall mechanical behavior practically equals that of a single mass-spring-damper system, of which the undamped eigenfrequency and the relative damping can be designed favorably. With respect to traditional voltage steering, charge steering has the advantage that no hysteresis is encountered between electrical input and elongation. Electrical steering configurations for both cases of steering are presented. Finally, for the case of charge steering, we derived the total model of a piezo-actuated positioning mechanism. This model is dominated by the mechanical model, which could be designed favorably. Therefore, this model gives a broad range of possibilities for model-based controller design.