Carbon aerogels are very promising substrates for electrocatalyst deposition involved in fuel cells. Their advantage over high surface area carbon blacks currently used, is the porous monolithic structure yielding large pore volumes with controlled pore sizes. By changing the synthesis parameters, it is possible to adjust their multi-scale structure which is strongly related to the electrochemical performances. The aim of the lecture is to give a survey of information about the multi-scale structure that can be obtained by small and wide angle X-ray scattering (SAXS and WAXS) techniques combined with contrast variation (CV). To this end, a series of SAXS experiments on carbon aerogels are described and the analysis of the experimental data is explained. Particular attention is paid to the determination of the specific surface area, SSAXS, and to the reasons why WAXS curves combined to SAXS ones make this determination more pertinent. The physical meaning of similarity or difference between SSAXS and surface area determined by gas adsorption, SADS, is discussed and information obtained by using contrast variation (CV) is described for two carbon aerogels prepared in different conditions.