I will present recent results on Deligne's Conjecture on special values of L-functions. I will discuss a proof of the analogue of Deligne's Conjecture for automorphic representations of GL(2n) and GL(n+1) x GL(n). The proof is based on the existence global rational structures on the space of regular algebraic cusp forms, which I will discuss first. Such rational structures arise from well known rational structures on the finite part of regular algebraic cuspidal representations as constructed by Clozel in the 90ies, and a new construction of rational structures on representations of real reductive Lie groups based on Zuckerman's cohomological induction, generalized to arbitrary fields of characteristic 0. Our applications to special values of L-functions rely on modular symbols and a delicate study of the rationality properties of archimedean zeta integrals.