The "multislice" method is used to simulate HREM images of gold particles. The 1024 × 1024 sampling of the supercells and the implementation on a CRAY 1S/2300 computer enable for the first time the accurate calculation of images of icosahedral aggregates containing up to 2869 atoms. The agreement with observations is very good for images along a threefold symmetry axis. For the fivefold symmetry, the computed contrasts clearly explain some anomalous observed features.