This book presents the mathematical theory of finite elements, starting from basic results on approximation theory and finite element interpolation and building up to more recent research topics, such as subgrid viscosity methods and Discontinuous Galerkin methods. The main body of the text is organized into three main sections. The first part develops the theoretical basis for the finite element method, emphasizing inf-sup conditions over the more conventional Lax-Milgrim paradigm, while the second and third parts address various applications and practical implementaions of the method, respectively.