Due to the likely ageing of the French population over the coming decades, the future financing of the current public,《 pay-as-you-go 》 pension system is quite worrisome. This article offers a framework for evaluating the long-run economic consequences of various possible scenarios: it uses an overlapping-generations, general equilibrium model of the French economy. After the description of the model, we present the results of two demographic variants, which clearly show that the current system would be sustainable in the long run only if population growth is positive. Finally the comparison of macroeconomic outcomes and the--individual and social--welfare analysis suggest that, if the forecast of a stagnating French population comes true, raising the legal retirement age would clearly dominate other alternatives such as curtailing benefits or switching to a pure capitalization scheme.