Mathematical Theory of Finite and Boundary Element Methods These are the lecture notes of the seminar Mathematische Theorie der finiten Element und Randelementmethoden organized by the Deutsche Mathematiker Vereinigung and held in Düsseldorf from June 7th 14th 1987. Finite element methods nowadays belong to the standard routines for the computation of solutions to boundary and initial boundary value problems of partial differential equations with many applications as e.g. in elasticity and thermoelasticity fluid mechanics acoustics electromagnetics scattering and diffusion. These methods also stimulated the development of corresponding mathematical numerical analysis. The seminar as well as these notes consist of three parts: I. An Analysis of the Finite Element Method for Second Order Elliptic Boundary Value Problems by A.H. Schatz II. On Finite Elements for Parabolic Problems by V. Thomée III. Boundary Element Methods for Elliptic Problems by W.L. Wendland. As a valuable addition to the standard literature in this field these notes provide an introduction and overview of the three different topics which shed light on recent research