This work summarizes the principles of Finite-Element-Modeling for bulk-acoustic-wave devices. It illustrates the model setup in terms of the governing system of equations and boundary conditions, as well as the efficient computer implementation. While some problems require the significant effort of 3D models, various effects can be addressed by fast and efficient 2D simulations. Multiple examples will be demonstrated of how numerical simulations can improve our understanding of the underlying physics, support the design of actual products, and help to pioneer the way into new technologies by facilitating to explore novel device concepts.