This chapter presents the set of governing equations to study the behavior of active materials that have an intrinsic ability for coupling several branches of physics and, consequently, are commonly used for manufacturing harvesters. Once the equations are defined, a numerical formulation based on the finite element method is developed in order tomodel these materials. In particular, this chapter studies the energy production from themechanical vibrations present in high-speed railway bridges. For this purpose, a review of the basic parameters of these bridges, their vibrations, frequencies and the dynamic characteristics are highlighted. Then, cantilever harvesters made out of piezoelectric and piezomagnetic materials are simulated under typical mechanical vibrations, and several conclusions are highlighted.