The Meso-NH Atmospheric Simulation System is a joint project by the Centre National de Recherches Météorologiques and Laboratoire d'Aérologie, designed to simulate atmospheric motions from the meso-alpha scale down to the micro-scale. The system includes a numerical model with a comprehensive physical package, an ensemble of facilities for preparing initial states, and a flexible post-processing and graphical facility. The model is based on the Lipp's and Hemler form of the anelastic system and allows for the simulation of multiple scales of motion through grid-nesting techniques. It supports the transport and diffusion of passive scalars and can be coupled with a chemical module. The model uses Fortran 90 and is designed to be easily implemented on UNIX machines. This paper presents the adiabatic formulation and basic validation simulations, along with a list of available physical parametrizations and initialization methods. The model's equations are formulated in a Cartesian frame and transformed into a stretched, terrain-following coordinate system to account for the Earth's sphericity and underlying topography. The discretization uses a C-grid in both horizontal and vertical directions, with second-order finite differences and two-point averages. The model offers various advection schemes, including a Multidimensional Positive Definite Centred Difference Scheme (MPDCD) that provides a good quality-cost ratio. The paper also discusses boundary conditions, coordinate systems, and grid stretching techniques.The Meso-NH Atmospheric Simulation System is a joint project by the Centre National de Recherches Météorologiques and Laboratoire d'Aérologie, designed to simulate atmospheric motions from the meso-alpha scale down to the micro-scale. The system includes a numerical model with a comprehensive physical package, an ensemble of facilities for preparing initial states, and a flexible post-processing and graphical facility. The model is based on the Lipp's and Hemler form of the anelastic system and allows for the simulation of multiple scales of motion through grid-nesting techniques. It supports the transport and diffusion of passive scalars and can be coupled with a chemical module. The model uses Fortran 90 and is designed to be easily implemented on UNIX machines. This paper presents the adiabatic formulation and basic validation simulations, along with a list of available physical parametrizations and initialization methods. The model's equations are formulated in a Cartesian frame and transformed into a stretched, terrain-following coordinate system to account for the Earth's sphericity and underlying topography. The discretization uses a C-grid in both horizontal and vertical directions, with second-order finite differences and two-point averages. The model offers various advection schemes, including a Multidimensional Positive Definite Centred Difference Scheme (MPDCD) that provides a good quality-cost ratio. The paper also discusses boundary conditions, coordinate systems, and grid stretching techniques.