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