This is the second volume of the proceedings of the Automorphic Semester held at Centre Emile Borel in Paris in 2000. It contains five important articles on various arithmetic aspects of automorphic forms on GSp(4): G. Laumon extends his earlier results on cohomology of Siegel 3-folds to the case of non-trivial coefficient systems; R. Weissauer constructs ℓ-adic Galois representations attached to discrete series automorphic forms GSp(4) and establishes some of their properties; E. Urban discusses the local behaviour of these representations at ℓ; A. Genestier and J. Tilouine prove a modularity lifting theorem for 4-dimensional symplectic Galois representations; D. Whitehouse proves a weighted twisted fundamental lemma necessary for establishing a functorial transfer of packets from GSp(4) to GL(4). The book will be of interest to researchers and graduate students interested in arithmetic applications of automorphic forms.