Introduction -- Statement of assumptions. Some properties of discrete groups satisfying the assumptions -- Definition of a cusp form (after Gelfand). Basic properties of cusp forms -- Definition of Eisenstein series. Investigation of the constant term in the Fourier expansion of an Eisenstein series. A variant of a formula of Selberg -- Some lemmas used in Sections 6 and 7 -- Proof of the function equations for the Eisenstein series associated to cusp forms -- Proof of the functional equations for all Eisenstein series. Statement of theorem -- References -- Appendices