Predicting dynamic behavior of nonlinear micromechatronic systems is rather difficult, because of the effects produced by electromechanical coupling, nonlinearities and microscale. This lecture is aimed to introduce basic methods for mechanical design of microelectromechanical systems (MEMs), by means of a test case consisting of an electrostatic microactuator, with few degrees of freedom. A model of the whole microsystem is built, by following a systematic approach, typical of structural mechatronics, dealing with formulation and solution of the equations of motion. Methods herewith presented are already extended to continuous microsystems, applied to numerical methods and implemented in some commercial and academic tools.