In this talk, I will present some results concerning the classification of the cuspidal automorphic representations of GL(n) over Q whose archimedean component is `` algebraic ‘’ and whose non archimedean components are all unramified.  As an application, I will sketch a proof of the following result : there are exactly 23 Siegel modular cusp forms of weight 12, for the full Siegel modular group, in genus at most 12.