We report a new variant of three-dimensional (3D) electron microscopy, Z-contrast tomography, in which, by use of a field-emission gun scanning transmission electron microscope equipped with a high-angle annular dark-field detector, images are formed by Rutherford scattered electrons. The intensity of such electrons is approximately proportional to Z(2) (Z being the atomic number of the scattering atom) so that the technique preferentially images high-Z materials distributed over low-Z supports. The 3D structure of nanoparticles catalysts, as well as that of the porous support (and especially the precise location of the former on the latter), is reconstructed from a tilt series of 2D projections by a (real space) tomography procedure using a weighted back projection approach.