This book is a report of a cognitive journey towards the reliable simulation of scattering problems using finite element methods. The pre-asymptotic analysis of Galerkin FEM for the Helmholtz equation with moderate and large wave number forms the core of the book. The topics include a priori and a posteriori error estimation as well as the stabilized methods. The mathematical investigation of model problems is given in close connection with the physical problem of acoustic scattering and fluid-solid interaction. Starting from the basic physical assumptions, the author methodically develops both the strong and weak forms of the governing equations. The main chapter on finite element analysis is preceded by a systematic treatment of Galerkin methods for indefinite sesquilinear forms. The book also contains broad reference material on numerical methods for the Helmholtz equation in unbounded domains, including Dirichlet-to-Neumann methods, absorbing boundary conditions, infinite elements and the perfectly matched layer. In the final chapter, three dimensional computational simulations are presented and compared with experimental data. Students and researchers in mathematics, physics and engineering, as well as scientists and computational engineers working on scattering problems will find this book of interest. The author has produced a self-contained and easily readable work containing numerous illustrations of the theory with numerical examples and computational results.