A deep problem at the intersection of number and group theory is the decomposition of the space L2 (G((Q)G(/A)), where G is a reductive group defined over (Q and /A is the ring of adeles of (Q. Langlands reduced this decomposition to that of the (smaller) spaces of cuspidal automorphic forms for certain subgroups of G. The present book describes this proof in detail. The starting point is the theory of automorphic forms, which can also serve as a first step towards understanding the Arthur-Selberg trace formula. To make the book reasonably self-contained, the authors have also provided essential background to subjects such as automorphic forms, Eisenstein series, Eisenstein pseudo-series (or wave-packets) and their properties. It is thus also an introduction, suitable for graduate students, to the theory of automorphic forms, written using contemporary terminology. It will be relevant to number theorists, representation theorists, and all whose work involves the Langlands program.