Fundamental to the design of intelligent material systems and structures are the realization of attributes of the constitutive transducer materials—the sensors and actuators—and the ability to model the characteristics of these transducers. In this thesis, electromechanical behaviors of the electrostrictive relaxor ferroelectric lead magnesium niobate-lead titinate (PMN-PT) are experimentally characterized and phenomenologically modeled. The dependencies of PMN-PT electromechanical transduction on temperature and frequency, characteristics of relaxor ferroelectrics, and on applied direct-current electric field, an attribute of electrostrictors which enables tunable transduction sensitivities, are investigated and modeled with respect to electrical, sensing, and actuation properties. A general procedure for using the developed constitutive models to quantitatively describe the behavior of PMN-PT is introduced for sensing and for the three types of actuation—servo, on/off, and alternating current.