Ultrasonic systems are usually comprised of three elements: a generator providing the electrical energy, an electromechanical transducer converting electrical energy into mechanical vibratory energy, and a propagation medium, fluid or solid, in which acoustic energy is radiated. These devices mainly utilize two mechanisms for energy conversion: an electromechanical transduction using piezoelectric effect and, when necessary, a mechanoacoustical conversion through the fluid–structure coupling. When the external fluid cannot be considered as a light fluid, a strong interaction takes place between electrical, mechanical, and acoustical phenomena. To design such systems, models must take into account both transduction mechanisms and arbitrary and complex geometries. Numerical methods are therefore an obvious choice. In numerical models, the generator is often considered as an ideal voltage (or current) generator delivering a sinusoidal electrical excitation at constant frequency. Detailed analysis of the electrical generator is generally not considered at the design stage. Hence, only the transducer, the propagation medium, and their interaction is considered in the model and are described by the finite element method (FEM), which can be coupled with other methods to describe unbounded media (boundary element method, Dirichlet-to-Neuman (DtN) method, acoustic dampers, perfectly matched layers (PML), etc.). This chapter describes numerical methods utilized to model piezoelectric devices. It is divided into three parts, dedicated, respectively, to transduction phenomena and wave propagation in solids, acoustic wave propagation in fluids and fluid–structure coupling, and acoustic radiation. The models are linear and rely upon the theory of elasticity, the constitutive equations of piezoelectricity, and the theory of linear acoustics. Application examples illustrate typical results and comparison with measurements.