The ability to visualize the three-dimensional structure of an object is of central importance in many branches of science and engineering. Recording an angular series of two-dimensional projections (images) and Subsequently applying tomographic reconstruction methods has proven to be extremely successful at many lengthscales. Using scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) imaging, we describe how the three-dimensional distribution of bimetallic nanocatalysts (such as Ru10Pt2, diameter similar to1 nm, mass 10(-21) g) within mesoporous hosts may be precisely determined. Moreover, the simultaneous retrieval of the elemental composition (from electron-stimulated X-ray emission spectra) of individual catalytic nanoparticles was achieved.