Determining the three-dimensional (3D) arrangement of atoms in crystalline nanoparticles is important for nanometre-scale device engineering and also for applications involving nanoparticles, such as optoelectronics or catalysis. A nanoparticle's physical and chemical properties are controlled by its exact 3D morphology, structure and composition 1 . Electron tomography enables the recovery of the shape of a nanoparticle from a series of projection images [2][3][4] . Although atomic-resolution electron microscopy has been feasible for nearly four decades, neither electron tomography nor any other experimental technique has yet demonstrated atomic resolution in three dimensions. Here we report the 3D reconstruction of a complex crystalline nanoparticle at atomic resolution. To achieve this, we combined aberration-corrected scanning transmission electron microscopy [5][6][7] , statistical parameter estimation theory 8,9 and discrete tomography 10,11 . Unlike conventional electron tomography, only two images of the target-a silver nanoparticle embedded in an aluminium matrix-are sufficient for the reconstruction when combined with available knowledge about the particle's crystallographic structure. Additional projections confirm the reliability of the result. The results we present help close the gap between the atomic resolution achievable in two-dimensional electron micrographs and the coarser resolution that has hitherto been obtained by conventional electron tomography.High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) is an imaging technique in which a focused electron probe is scanned across an electron-transparent sample 12 .Using an annular-shaped high-angle detector behind the sample, the signal is dominated by Rutherford and thermal diffuse scattering. When applied to a nanocrystal in zone-axis orientation, the HAADF signal approximately scales with the square of the atomic number Z and with the thickness of the sample [13][14][15] . By using aberration-corrected probe forming optics [5][6][7] , a resolution of the order of 50 picometres can nowadays be demonstrated 16 . Therefore, it is generally believed that aberration-corrected HAADF STEM has the potential to achieve atomic resolution in three dimensions. Electron tomography is the most common approach used to reconstruct nanomaterials in three dimensions. The 3D reconstruction is computed from a tilt series of projection images acquired while rotating the sample. Spatial resolution for the reconstruction is around one cubic nanometre 2-4 , limiting its use for attaining atomic resolution. Another potential technique with which to obtain 3D structure information is 'depth-sectioning', in which a sample is optically sliced by changing the objective lens focus 17 . Single atoms can be visualized using this technique 18 , yet 3D reconstructions at atomic resolution have not been demonstrated because the depth resolution is insufficient to resolve interatomic distances along the optical axis. Discrete tomography, a recons...

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