The focus of this book is concerned with the modeling and precise numerical simulation of mechatronic sensors and actuators. These sensors, actuators, and sensor - actuator systems are based on the mutual interaction of the me­ chanical field with a magnetic, an electrostatic or an electromagnetic field. In many cases the transducer is immersed in an acoustic fluid and the solid-fluid coupling has to be taken into account. Examples are: piezoelectric stack ac­ tuators for common-rail injection systems, micromachined electrostatic gyro sensors used in stabilizing systems of automobiles or ultrasonic imaging sys­ tems for medical diagnostics. The modeling of mechatronic sensors and actuators leads to so-called mul­ tifield problems, which are described by a system of nonlinear partial dif­ ferential equations. Such systems can not be solved analytically and, thus a numerical calculation scheme has to be applied. The schemes discussed in this book are based on the finite element (FE) method, which is capable of efficiently solving the partial differential equations. The complexity of the simulation of multifield problems consists in the simultaneous computation of the involved single fields as well as in the coupling terms, which intro­ duce additional nonlinearities. Examples are: moving conductive (electrically charged) body within a magnetic (an electric) field, electromagnetic and/or electrostatic forces.