Characterisation of fuel cell catalyst materials using electron tomography. In order to optimize catalysts for fuel cells they are characterized by electron tomography. This method provides unique insights into the nanometre structures of the metallic catalyst particles deposited on an electrically conducting inert carbon support. The threedimensional imaging method does not only allow for qualitative investigations, but also for detailed quantitative shape and structure analysis of the catalyst. For example the positions of the catalytic particles are analysed with respect to the support material. Their shape and embedding into the support are evaluated which substantially define the technologically important free surface of the catalytic particles. Electron tomography therefore enables quantitative comparison between catalytic materials from different preparation processes or production lines. It contributes to decipher the relationship between electrocatalytic properties and structural features of the nano scaled materials and is therefore a base for further optimization of the catalyst materials