The increasing use of piezoelectric material in the last decade has led to the new discovery that piezoelectric devices may be used not only as sensors or actuators individually, but also as a sensor and actuator at the same time. This bi-directional phenomenon greatly expands the utility of piezoelectric devices, especially in the area of smart material systems and structures, such as structural dynamic measurement analysis, structural damage detection, active structural vibration and acoustic control, etc. Presented in this thesis is a new electromechanical approach for structural dynamic analysis. It uses PZT patches bonded on structures as active piezoelectric sensors, and acquires the structural information by measuring the electric admittance of the sensors. Because of the electromechanical coupling, the electric admittance is mechanically modulated by the structural dynamics through the piezoelectric effect. The structural dynamic characteristics can then be extracted using a mathematical model governing the interaction of PZT actuators and structures. In this thesis, a multi-input, multi-output mathematical model is derived based on the one-dimensional constitutive law of the PZT and the mechanical impedance approach. The model yields explicit expressions of structural mechanical mobility in terms of the measured electric admittance. Applications of the approach in smart structures are presented in the area of mechanical frequency response function extraction, structural modal analysis, and structural health monitoring. As a general issue related to the application of PZT sensor-actuators in smart structures, the energy conversion efficiency of PZT actuators has also been experimentally investigated. / Master of Science