 The Meso-NH Atmospheric Simulation System is a joint effort between the Centre-National de Rocherche's meteorologiques and laboratory derology. It includes a numerical model, physical package, flexible file manager, ensemble of initial states, post-processing facility, interactive procedures, and Fortran-90 compiler. The system is designed as a research tool for small and meso-scale atmospheric processes and is freely accessible to the research community. The present paper provides a general description of the adiabatic formulation and basic validation simulations, along with a list of physical parameterizations and initialization methods. A more detailed description will be provided in a future paper. This article was authored by J.P. Lafour, J. Stein, N. Asensio, and others. We are article.tv, links in the description below.