To construct models and to apply quantitative techniques in order to explain the cyclical movements of the economy is one of the main aims of “X-Crise” group (nickname of “Centre Polytechnicien d'Etudes Economiques”) at the “Ecole Polytechnique” in Paris. Indeed, french polytechniciens' engineers hope that mathematical economics, and especially empirically based modelization, will be helpful first to build a true economic science, and second, to find solutions to the 1930's crisis. These hopes are developed in the methodological debate that Polytechniciens began even before the creation of the association X-Crise. They explain in particular their rejection of "pure” economics and their defence of an approach which mixes economic concepts, statistical facts and mathematical model – such as econometrics. Overall, these hopes are expressed through models constructed by X-Crise members, such as Polytechniciens like François Moch and Maurice Potron or non-polytechniciens like brothers Georges and Edouard Guillaume. Attempts were done to include business cycle in their models. But, finally, they failed to confront them to empirical data.