This book presents a unified theory of the Finite Element Method and the Boundary Element Method for a numerical solution of second order elliptic boundary value problems. While there are textbooks on the finite element method, this is one of the first books on Theory of Boundary Element Methods. The book includes the solvability, stability, and error analysis as well as efficient methods to solve the resulting linear systems. Applications are the potential equation, the system of linear elastostatics and the Stokes system. The book is suitable for self study and exercises are included.