The response of five commercial piezoelectric stack actuators under electrical, mechanical, and combined electro-mechanical loading was investigated in this study. The focus was to understand the behavior of piezoelectric materials under the combined electro-mechanical loading scenario, and to determine fundamental properties important for design of actuator systems that incorporate these materials. Parameters that were evaluated include strain output, permittivity, mechanical stiffness, energy density, and coupling coefficients as a function of mechanical preload and electric field values representative of in- service conditions. Stiffness measurements indicate strong dependence on applied electric field and mechanical preload, as well as the number of mechanical cycles. For certain actuators, stiffness values change by as much as 100% depending on the operating conditions. The voltage induced strain output of some of these samples which include both PZT and PLZT compositions exceeds 2,000 microstrain for certain operating conditions (under the constant preload). Initially, the strain output is enhanced with an increase in mechanical preload and the maximum strain values are obtained when the stacks are preloaded between 4 - 6 ksi. High values of output energy density can be achieved for this operating region. Applying a higher preload has the advance effect on the stacks response since mechanical loading impedes domain wall motion reducing the overall strain output. A similar effect is observed under the combined out-of-phase electro-mechanical loading, and we have found that the highest energy density is obtained if the mechanical loading amplitude does not exceed +/- 2.5 ksi.