The Meso-NH Atmospheric Simulation System is a joint effort between the Centre National de Recherches Météorologiques and the Laboratoire d'Aérologie. It includes a numerical model capable of simulating atmospheric motions from large-scale to micro-scale, with a comprehensive physical package, flexible file management, and tools for initial state preparation and post-processing. The model is based on the Lipps and Hemler form of the anelastic system, allowing for the simulation of multiple scales through interactive grid-nesting. It supports the transport and diffusion of passive scalars coupled with a chemical module and uses Fortran 90. The model is designed for research on small and meso-scale atmospheric processes, is freely accessible, and user-friendly. The paper presents the adiabatic formulation and validation simulations, along with available physical parametrizations and initialization methods. The model uses a Cartesian coordinate system with a reference state based on the Lipps and Hemler anelastic approximation. It includes equations for continuity, momentum, thermodynamics, and conservation of moisture and passive scalars. The model also features a pressure problem solution and various boundary conditions. The system uses a C-grid for discretization and includes a positive definite advection scheme. The model is designed for flexibility, portability, and maintainability, and is intended for use in research and operational forecasting.The Meso-NH Atmospheric Simulation System is a joint effort between the Centre National de Recherches Météorologiques and the Laboratoire d'Aérologie. It includes a numerical model capable of simulating atmospheric motions from large-scale to micro-scale, with a comprehensive physical package, flexible file management, and tools for initial state preparation and post-processing. The model is based on the Lipps and Hemler form of the anelastic system, allowing for the simulation of multiple scales through interactive grid-nesting. It supports the transport and diffusion of passive scalars coupled with a chemical module and uses Fortran 90. The model is designed for research on small and meso-scale atmospheric processes, is freely accessible, and user-friendly. The paper presents the adiabatic formulation and validation simulations, along with available physical parametrizations and initialization methods. The model uses a Cartesian coordinate system with a reference state based on the Lipps and Hemler anelastic approximation. It includes equations for continuity, momentum, thermodynamics, and conservation of moisture and passive scalars. The model also features a pressure problem solution and various boundary conditions. The system uses a C-grid for discretization and includes a positive definite advection scheme. The model is designed for flexibility, portability, and maintainability, and is intended for use in research and operational forecasting.