EMS is a set of computer programs which has been developed not only for the simulation and analysis of High-Resolution Electron Microscopy images, but also for the analysis of diffraction patterns. It is composed of small, dedicted tasks which have a well defined function. Schematically EMS can be divided into two parts. The first part is able to handle the typical calculations of crystallography encountered in electron microscopy such as automatic indexing of diffraction and Kikuchi patterns, simulation of spot and powder patterns, drawing of stereographic projections, calculation of the Bravais lattice of an unknown structure from two diffraction patterns, etc. The second part offers the possibility of the simulation of structure images of both perfect and defect structures using either a Fast-Fourier-Transform-based multislice approach or the Bloch wave formalism. The dialogue with EMS is “menu”-driven and completely interactive. Moreover numerous “note” and “help” files are available on-line for the convenience of the user.