The main subject of the book is the arithmetic of zeta functions of automorphic forms. More precisely, it looks at p-adic properties of the special values of these functions. For the Riemann-zeta function this goes back to the classical Kummer congruences for Bernoulli numbers and their p-adic analytic continuation of the standard zeta functions of Siegel and modular forms and of the convolutions of Hilbert modular forms. The book is addressed to specialists in representation theory, functional analysis and algebraic geometry. Together with new results, it provides considerable background information on p-adic measures, their Mellin transforms, Siegel and Hilbert modular forms, Hecke operators acting on them, and Euler products.